"Maintainer" will be freed to be used for #42751

When a project is not private, and the source branch not protected the
user can now select the option to allow maintainers to push to this
branch

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

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

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38389

Previously the merge request widget would use the `updated_at` date as
the tooltip text for both closed & merged states. This is incorrect as
the `updated_at` date is actually changed when a user updates merge
request through commenting, description changes or anything else.

The widget states for merged & closed events now use their own event
object which holds their own `updated_at` date string. Also this text
has been correctly formatted through our date utilities to correctly
display the right timezone data in a user friendly way.

Closes #38545

See https://gitlab.com/gitlab-org/gitlab-ce/issues/20076

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33287

The MR widget was trying to render the pipelines section when
there are no GitLab CI pipelines which was throwing some NPE
errors.

Having two states that essentially mean the same thing is very much like
having a boolean "true" and boolean "mostly-true": it's rather silly.
This commit merges the "reopened" state into the "opened" state while
taking care of system notes still showing messages along the lines of
"Alice reopened this issue".

A big benefit from having only two states (opened and closed) is that
indexing and querying becomes simpler and more performant. For example,
to get all the opened queries we no longer have to query both states:

    SELECT *
    FROM issues
    WHERE project_id = 2
    AND state IN ('opened', 'reopened');

Instead we can query a single state directly, which can be much faster:

    SELECT *
    FROM issues
    WHERE project_id = 2
    AND state = 'opened';

Further, only having two states makes indexing easier as we will only
ever filter (and thus scan an index) using a single value. Partial
indexes could help but aren't supported on MySQL, complicating the
development process and not being helpful for MySQL.

This reverts commit 6ac777a7, reversing
changes made to 025cbc2a.