Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.

This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.

Only the reason with the most priority gets sent.

check project access on MR create

See merge request gitlab/gitlabhq!2273

(cherry picked from commit 1fe2325d6ef2bced4c5e97b57691c894f38b2834)

43e85f49 check project access on MR create

Validate project path in Gitlab import - 10.3 port

See merge request gitlab/gitlabhq!2268

(cherry picked from commit 94c82376d66fc80d46dd2d5eeb5bade408ec6a7e)

2b94a7c2 Validate project path in Gitlab import

Remove order param from the MilestoneFinder

See merge request gitlab/gitlabhq!2259

(cherry picked from commit 14408042e78f2ebc2644f956621b461dbfa3d36d)

155881e7 Remove order param from the MilestoneFinder

Closes #41633

Closes #33423

Closes gitaly#863

Instead of storing detailed rebase error, only a generic message is
stored with MR. The reason is that this message is exposed and displayed
to end user and there is no reason to expose detailed backend
information. Error message is still logged so detailed information
can be found in logfile by admin if needed.

Related #41820

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

We'd need to keep track of project full path otherwise directory tree created with hashed storage enabled cannot be usefully imported using the import rake task.

In this case the project will get the minimum between both
visibilities.

If that visibility is restricted, then a lower level will be picked.