When using IssuableFinder/IssuesFinder to find issues for multiple
projects it's more efficient to use a JOIN + a "WHERE project_id IN"
condition opposed to running a sub-query.

This change means that when finding issues without labels we're now
using the following SQL:

    SELECT issues.*
    FROM issues
    JOIN projects ON projects.id = issues.project_id

    LEFT JOIN label_links ON label_links.target_type = 'Issue'
                          AND label_links.target_id  = issues.id

    WHERE (
        projects.id IN (...)
        OR projects.visibility_level IN (20, 10)
    )
    AND issues.state IN ('opened','reopened')
    AND label_links.id IS NULL
    ORDER BY issues.id DESC;

instead of:

    SELECT issues.*
    FROM issues
    LEFT JOIN label_links ON label_links.target_type = 'Issue'
                          AND label_links.target_id  = issues.id

    WHERE issues.project_id IN (
        SELECT id
        FROM projects
        WHERE id IN (...)
        OR visibility_level IN (20,10)
    )
    AND issues.state IN ('opened','reopened')
    AND label_links.id IS NULL
    ORDER BY issues.id DESC;

The big benefit here is that in the last case PostgreSQL can't properly
use all available indexes. In particular it ends up performing a
sequence scan on the "label_links" table (processing around 290 000
rows). The new query is roughly 2x as fast as the old query.

Since this method's returned data doesn't change between calls on the
same IssuableFinder instance we can just memoize this similar to the
"project" method.

This field is queried when filtering issues and due to the lack of an
index would end up triggering a sequence scan.

Improve performance of user profiles, finding groups, and finding projects

This MR improves the following:

* Rendering of profile pages and Atom feeds
* Finding groups (using GroupsFinder & friends)
* Finding projects (using ProjectsFinder & friends)

Initially this MR was intended to only improve rendering of Atom feeds, but over time other fixes were introduced as well as the same code was the cause of all these problems.

See merge request !1790

Check which folders and archives should be packed before passing to tar command.

If user uses backup task with SKIP and skips one of the archives listed(uploads, builds, artifacts) backup create will give an error: `Cannot stat: No such file or directory`.

This MR fixes that by checking for skipped items.
Additionally, compact everything to avoid `TypeError: no implicit conversion of nil into String` errors.

See merge request !1824

Fix huge line height for diff files list
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1826

Backport relevant changes from gitlab-org/gitlab-ee!51

To do:

- [x] Update gitlab-shell

See merge request !1822

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

When getting the projects of a user we should get the public _and_
internal projects, not just the public ones.

Apparently MySQL doesn't support this syntax:

    (...) UNION (...)

instead it only supports:

    ...
    UNION
    ...

This won't work efficiently if you happen to have a lot of projects.

These changes are based on those from commit
03f5ff75, except they use a UNION
instead of plucking IDs into memory.

Previously this controller would in multiple places load tons (read:
around 65000) project and/or group IDs into memory. These changes in
combination with the previous commits significantly cut down loading
times of user profile pages and the Atom feeds of users.

These methods no longer include public groups/projects (that don't
belong to the actual user) as this is handled by the various finder
classes now. This also removes the need for passing extra arguments.

Note that memoizing was removed _explicitly_. For whatever reason doing
so messes up the users controller to a point where it claims a certain
user does _not_ have access to certain groups/projects when it does have
access. Existing code shouldn't be affected as these methods are only
called in ways that they'd run queries anyway (e.g. a combination of
"any?" and "each" which would run 2 queries regardless of memoizing).

This method can be used to filter projects to those visible to a given
user.

These methods will be used to get a list of groups, optionally
restricted to only those visible to a given user.

This will be used to move some querying logic from the users controller
to the Event model (where it belongs).

This class now uses a UNION (when needed) instead of plucking tens of
thousands of project IDs into memory. The tests have also been
re-written to ensure all different use cases are tested properly
(assuming I didn't forget any cases).

The finder has also been broken up into 3 different finder classes:

* ContributedProjectsFinder: class for getting the projects a user
  contributed to.
* PersonalProjectsFinder: class for getting the personal projects of a
  user.
* ProjectsFinder: class for getting generic projects visible to a given
  user.

Previously a lot of the logic of these finders was handled directly in
the users controller.

In the previous setup the GroupsFinder class had two distinct tasks:

1. Finding the projects user A could see
2. Finding the projects of user A that user B could see

Task two was actually handled outside of the GroupsFinder (in the
UsersController) by restricting the returned list of groups to those the
viewed user was a member of. Moving all this logic into a single finder
proved to be far too complex and confusing, hence there are now two
finders:

* GroupsFinder: for finding groups a user can see
* JoinedGroupsFinder: for finding groups that user A is a member of,
  restricted to either public groups or groups user B can also see.

This new setup no longer loads any IDs into memory using "pluck",
instead using SQL UNIONs to merge the various datasets together. This
results in greatly improved query performance as well as a reduction of
memory usage.

The old setup was in particular problematic when requesting the
authorized projects _including_ public/internal projects as this would
result in roughly 65000 project IDs being loaded into memory. These IDs
would in turn be passed to other queries.

This removes the need for plucking any IDs into Ruby.

By moving the default sort order into a separate scope (and calling this
from the default scope) we can more easily re-apply a default order
without having to specify the exact column/ordering all over the place.

This allows retrieving of the list of authorized projects using a single
query, without having to load any IDs into Ruby. This in turn also means
we can remove the method User#authorized_projects_id.

This class can be used to join multiple AcitveRecord::Relation objects
together using a SQL UNION statement. ActiveRecord < 5.0 sadly doesn't
support UNION and existing Gems out there don't handle prepared
statements (e.g. they never incremented the variable bindings).