Fix error 500 in Todos

Closes #14095

Closes #14075

Closes #14109

Closes #14151

See merge request !3141

Branded appearance to CE

Closes #11489

The difference with the EE version is only that there is no distinction between light and dark logos, though this wasn't used anyway. If this is fine, I'll create a MR on EE too.

TODO:
- [x] Copy docs
- [x] Make new screenshots
- [ ] Remove Custom Welcome message feature?

@rymai: I was unsure what labels to add to ping you, so I just ping you like this 

/cc @DouweM

See merge request !2927

Add Todos

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/2425

Tasks:

-  Prepare database
 - [X] Create a new table (`todos`)
- Tasks Queue view
 - [X] Add a number icon showing the number of todos on the top right next to the new and logout button that will redirect the user to the todos page
 - [X] Add a chronological list of todos, with the 'Todos' tab active by default
 - [X] Add a 'Done' button to each todo
 - [x] Add filters (project, author, type, and action)
- Todos generation
 - [X] When user issue/mr is assgined to someone
 - [x] When user is mentioned on (issues/mr's/comments)
- Mark todo as `done`
 - [X] When clicks on the 'Done' button
 - [X] When edit issue/mr
 - [X] When left/edit a comment
 - [X] When reassign issue/mr
 - [X] When add/remove labels to issue/mr
 - [X] When issue/mr is closed
 - [X] When mr is merged
 - [X] When added an emoji
 - [X] When changed the issue/mr milestone

* Screenshot:

![Screenshot_2016-02-20_12.45.57](/uploads/4b2554b1bde25aed3347e1ae41e8e0c0/Screenshot_2016-02-20_12.45.57.png)

See merge request !2817

Add ability to revert changes introduced by Merge Requests or Commits

Closes #3409 

See merge request !1990

Make it possible to erase build content (artifacts, trace)

This feature makes it possible to remove build content - build artifacts and build traces.

- [x] Remove artifacts
- [x] Remove metadata
- [x] Remove build traces
- [x] Wait for https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1942 this to be merged
- [x] Fix the permissions after the merge

Closes #3421

See merge request !2560

Add the `read_build` ability if user is anonymous or guest and allow_guest_to_access_builds is enabled.

Currently any spam detected by Akismet by non-members via API will be logged
in a separate table in the admin page.

Closes #5612

This partially reverts 6a5cd3ca - we keep the migration and add a new
migration that reverts it in order to keep migration history intact.

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

Artifacts metadata field will be used to store a filename of gzipped
file containing metadata definition for given artifacts archive.

InfluxDB over UDP doesn't use authentication, thus there's no need for
these settings.

This removes the need for Sidekiq and any overhead/problems introduced
by TCP. There are a few things to take into account:

1. When writing data to InfluxDB you may still get an error if the
   server becomes unavailable during the write. Because of this we're
   catching all exceptions and just ignore them (for now).
2. Writing via UDP apparently requires the timestamp to be in
   nanoseconds. Without this data either isn't written properly.
3. Due to the restrictions on UDP buffer sizes we're writing metrics one
   by one, instead of writing all of them at once.