This removes all usage of soft removals except for the "pending delete"
system implemented for projects. This in turn simplifies all the query
plans of the models that used soft removals. Since we don't really use
soft removals for anything useful there's no point in keeping it around.

This _does_ mean that hard removals of issues (which only admins can do
if I'm not mistaken) can influence the "iid" values, but that code is
broken to begin with. More on this (and how to fix it) can be found in
https://gitlab.com/gitlab-org/gitlab-ce/issues/31114.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37447

When a project uses fast-forward merging strategy user has
to rebase MRs to target branch before it can be merged.
Now user can do rebase in UI by clicking 'Rebase' button
instead of doing rebase locally.

This feature was already present in EE, this is only backport
of the feature to CE. Couple of changes:
* removed rebase license check
* renamed migration (changed timestamp)

Closes #40301

In a previous attempt (rolled back in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16021) we tried
to migrate `issues.closed_at` from timestamp to timestamptz using a
regular migration. This has a bad impact on GitLab.com and as such was
rolled back.

This commit re-implements the original migrations using generic
background migrations, allowing us to still migrate the data in a single
release but without a negative impact on availability.

To ensure the database schema is up to date the background migrations
are performed inline in development and test environments. We also make
sure to not migrate that that doesn't need migrating in the first place
or has already been migrated.

This change is required because otherwise if a user is created with a
value for `projects_limit` that matches the DB default, it gets
overwritten by `current_application_settings.default_projects_limit`. By
removing the default we once again can allow a user to be created with a
limit of 10 projects without the risk that it'll change to 10000

Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.

To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.

Because the checks are now done outside normal request, we can have a
simpler failure strategy:

The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.

When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.

After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.

Two things at ones, as there was no clean way to seperate the commit and
give me feedback from the tests.

But the model Artifact is now JobArtifact, and the table does not have a
type anymore, but the metadata is now its own model:
Ci::JobArtifactMetadata.

To allow jobs/builds to have multiple artifacts, and to start seperating
concerns from Ci::Build a new model is created: Ci::Artifact.

Changes include the updating of the ArtifactUploader to adapt to a
slightly different interface. The uploader expects to be initialized
with a `Ci::Build`.

Futher a migration with the minimal fields, the needed foreign keys and
an index.

Last, the way this works is by prepending a module to Ci::Build so we
can basically override behaviour but if needed use `super` to get the
original behaviour.

This will prevent our other migration for adding old files to the uploads table from breaking.

Because it is a temporary table meant only to facilitate a migration of data. It is referenced only by the post-deploy migration and 2 related background migrations.

It should be dropped when the data migration is finished.

To create the table, and schedule the background migration that begins the work.

Rescheduling will make sure the fork networks with a deleted source
project are created.

For getting the SHAs from an MR to find pipelines, we get the last 100 MR diffs
for the MR, and find commits from those. This was un-indexed before, because the
index was not a composite index on merge_request_diff_id, id. Changing that
means that this scope can exclusively use indexes.

The st_commits and st_diffs columns on merge_request_diffs historically held the
YAML-serialised data for a merge request diff, in a variety of formats.

Since 9.5, these have been migrated in the background to two new tables:
merge_request_diff_commits and merge_request_diff_files. That has the advantage
that we can actually query the data (for instance, to find out how many commits
we've stored), and that it can't be in a variety of formats, but must match the
new schema.

This is the final step of that journey, where we drop those columns and remove
all references to them. This is a breaking change to the importer, because we
can no longer import diffs created in the old format, and we cannot guarantee
the export will be in the new format unless it was generated after this commit.

This ensures that merge_requests.state and merge_requests.merge_status
both have a proper default value and NOT NULL constraint on database
level. We also make sure to update any bogus rows first, without blowing
up the database.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/40534