This migration will take all nested groups and convert them into regular
groups, ensuring that members of any parent groups still have access to
the child group. This migration relies on code external to it as copying
all of this over involves hundreds of lines of code depending on all
sorts of methods, making this practically impossible to do right.

This hides/disables some UI elements and API parameters related to
nested groups when MySQL is used, since nested groups are not supported
for MySQL.

This commit introduces the usage of Common Table Expressions (CTEs) to
efficiently retrieve nested group hierarchies, without having to rely on
the "routes" table (which is an _incredibly_ inefficient way of getting
the data). This requires a patch to ActiveRecord (found in the added
initializer) to work properly as ActiveRecord doesn't support WITH
statements properly out of the box.

Unfortunately MySQL provides no efficient way of getting nested groups.
For example, the old routes setup could easily take 5-10 seconds
depending on the amount of "routes" in a database. Providing vastly
different logic for both MySQL and PostgreSQL will negatively impact the
development process. Because of this the various nested groups related
methods return empty relations when used in combination with MySQL.

For project authorizations the logic is split up into two classes:

* Gitlab::ProjectAuthorizations::WithNestedGroups
* Gitlab::ProjectAuthorizations::WithoutNestedGroups

Both classes get the fresh project authorizations (= as they should be
in the "project_authorizations" table), including nested groups if
PostgreSQL is used. The logic of these two classes is quite different
apart from their public interface. This complicates development a bit,
but unfortunately there is no way around this.

This commit also introduces Gitlab::GroupHierarchy. This class can be
used to get the ancestors and descendants of a base relation, or both by
using a UNION. This in turn is used by methods such as:

* Namespace#ancestors
* Namespace#descendants
* User#all_expanded_groups

Again this class relies on CTEs and thus only works on PostgreSQL. The
Namespace methods will return an empty relation when MySQL is used,
while User#all_expanded_groups will return only the groups a user is a
direct member of.

Performance wise the impact is quite large. For example, on GitLab.com
Namespace#descendants used to take around 580 ms to retrieve data for a
particular user. Using CTEs we are able to reduce this down to roughly 1
millisecond, returning the exact same data.

== On The Fly Refreshing

Refreshing of authorizations on the fly (= when
users.authorized_projects_populated was not set) is removed with this
commit. This simplifies the code, and ensures any queries used for
authorizations are not mutated because they are executed in a Rails
scope (e.g. Project.visible_to_user).

This commit includes a migration to schedule refreshing authorizations
for all users, ensuring all of them have their authorizations in place.
Said migration schedules users in batches of 5000, with 5 minutes
between every batch to smear the load around a bit.

== Spec Changes

This commit also introduces some changes to various specs. For example,
some specs for ProjectTeam assumed that creating a personal project
would _not_ lead to the owner having access, which is incorrect. Because
we also no longer refresh authorizations on the fly for new users some
code had to be added to the "empty_project" factory. This chunk of code
ensures that the owner's permissions are refreshed after creating the
project, something that is normally done in Projects::CreateService.

Allow numeric values in gitlab-ci.yml

Closes #30017

See merge request !10607

Fix adding defaults for concurrent column renames

See merge request !11335

Added balsamiq jasmine integration test

Closes #31889

See merge request !11257

add username field to push webhook

Closes #32136

See merge request !11272

Fix typo in code snippet to match text below

See merge request !11101

Fix the GitLab EE link

See merge request !11021

fix $CI_JOB_ID variable

See merge request !10966

Fetch commit deltas from Gitaly

Closes gitaly#199

See merge request !11122

Ensure schedule_form and schedules_index both utilize common_vue bundle

See merge request !11274

Closes gitaly#199

Backport New SystemHook: `repository_update`

Closes #26325

See merge request !11140

[ci skip]

[ci skip]

Wait for requests after each Spinach scenario instead of feature

See merge request !11349

[ci skip]

[ci skip]

Link to a case where we can't support MySQL

Closes #32181

See merge request !11334

Remove trailing comma in dependency linker to satisfy Rubocop

See merge request !11348

Add fixed positioning to top level navbar

Closes #30975

See merge request !10751

Relax rake backup regex to handle CE and EE RCs

See merge request !11345

Fix conflict resolution from corrupted upstream

Closes gitlab-ee#2128

See merge request !11298

New branch new mr docs

See merge request !11308

See http://rubular.com/r/uTDlT85zW1

I don't know why this happens exactly, but given an upstream and fork repository
from a customer, both of which required GC, resolving conflicts would corrupt
the fork so badly that it couldn't be cloned.

This isn't a perfect fix for that case, because the MR may still need to be
merged manually, but it does ensure that the repository is at least usable.

My best guess is that when we generate the index for the conflict
resolution (which we previously did in the target project), we obtain a
reference to an OID that doesn't exist in the source, even though we already
fetch the refs from the target into the source.

Explicitly setting the source project as the place to get the merge index from
seems to prevent repository corruption in this way.

Usage ping updates

Closes #31741 and #31453

See merge request !11231