Before, this would:

1. Not use the correct reference for non-JIRA external trackers.
2. Append 'Closes ' if an external tracker was enabled, but no issue matched the
   branch name.

Adds i18n
Improves test cases

Closes #38135.

This essentially allows to pass both user and email, so that we can
either prefer the user to retrieve the avatar or (if user is not
present) fall back to the email lookup.

Rather radical but avoids n+1 queries in the rather common case we want
to include information about the author.

We want to drop the generic #avatar_icon helper that supports both an email
and a user object being passed in. Instead, we want to explicitly use the #avatar_icon_for_user
and #avatar_icon_for_email helpers depending on what we have at hand.
This allows us to avoid unnecessary database queries (e.g. call
User.find_by_any_email if we already have the user).

In situations like here, this makes it less convenient to use.

We need to keep the email lookup as we want to fall back to displaying
the gravatar result for a given email if the user does not exist.

Whenever we already deal with a User object, let's use the more specific
method avatar_icon_for_user.

When we don't use the original `ApplicationHelper#avatar_icon` anymore, we
can just remove it (and its specs).

Closes #42800.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/43198

Closes #26466.

Especially making the link on Lock MR section work the same as others on hover

Closes #37436
Closes gitaly#998

When JIRA or Redmine were enabled and the branch name did not match the
matching regular expression, the `issue_iid` would be `nil`, preventing
users from creating merge requests.

Closes #43193