Fix 500 when selecting a mirror user

Closes gitlab-ce#29316

See merge request !1404

Add detailed explanation of merge options

Closes #257

See merge request !1350

When selecting a mirror user, we do a permissions check, which means
that `@users` is an array, not an ActiveRecord relation.

We also can't use `Array#delete`, because it uses `Object#equal?`, while
we need to be able to take advantage of ActiveRecord objects overriding
`#eql?` and `#hash?` (as `Array#uniq` uses a hash internally).

Add update doc for CE 9.0 to EE 9.0

See merge request !1394

Update storage settings to allow extra values per shard

See merge request !1362

Add Upload count to usage data

See merge request !1401

CE upstream

Closes gitaly#109 and gitlab-ce#28515

See merge request !1371

# Conflicts:
#   doc/api/v3_to_v4.md

Signed-off-by: Rémy Coutable <remy@rymai.me>

Port of 1381-present-commits-pagination-headers-correctly to EE

See merge request !1357

Resolve "Squash commits fails for branches named HEAD"

Closes #1861

See merge request !1391

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Port of 26202 change dropdown style slightly to EE

See merge request !1380

This would print an ambiguous refs warning, so silence that with
--quiet. HEAD in this context will refer to the 'current commit', not
the ref with that name.

Porting IssueBoardSorting feature from CE

See merge request !1388

Code improvements

See merge request !1385

This will be necessary when adding gitaly settings. This version
doesn't make any functional changes, but allows us to include this
breaking change in 9.0 and add the needed extra settings in the future
with backwards compatibility

Allow all parameters of group webhooks to be set through the UI

Closes #1853

See merge request !1376

This would fail after an AR connection was established by
8_metrics.rb (to load the application settings), because it would try to
render a route. For now, just ignore the problem by not creating a link
- the link is less useful in console output anyway.

Speed up MergeRequest::RefreshService spec by combining examples

See merge request !1386

Signed-off-by: Rémy Coutable <remy@rymai.me>

This reverts commit 685b0056.

Fix edit last visual token

See merge request !1383

Fix filtered search visual token editing dropdown

See merge request !1384