set proper signature marker in mails

Closes #43617

See merge request gitlab-org/gitlab-ce!17979

Add more detail to cleanup steps for background migrations

See merge request gitlab-org/gitlab-ce!18448

Fix regression around new SHA widget in "New Merge Request" Screen

Closes #45446

See merge request gitlab-org/gitlab-ce!18422

Improve web IDE commit input

Closes #44832

See merge request gitlab-org/gitlab-ce!18389

Ensure that cached_markdown_version is handled correctly for CommonMark

See merge request gitlab-org/gitlab-ce!18431

[Rails5] Add `safe_params` helper

See merge request gitlab-org/gitlab-ce!18241

1. We can't just steal from the queue, in case there was a problem with Sidekiq.
2. We need to consider import / export.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>

use rfc 3676 mail signature delimiters ("-- ") instead of the previous
"---" one.

closes #43617
Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>

This reverts merge request !18399