Enable arbitration in MailRoom

Fixes #2870 (closed).

@marin Don't worry about adding this to omnibus yet, I'll be moving all of the mail_room configuration to config/gitlab.yml so that source installation users don't need to edit config/mail_room.yml which contains a lot of stuff they shouldn't touch. See https://github.com/tpitale/mail_room/pull/53.

Reassigned to @rspeicher

Added 1 commit:

• c2c8f8ac - Remove warning about Reply by email and HA

@DouweM I see that your PR was merged and a new version was released. Do you want to roll that into this MR?

@rspeicher Nah, I'll post a separate MR. You can merge this one.

mentioned in merge request !1580 (merged)

@rspeicher It's in !1580 (merged).

Status changed to merged

mentioned in commit 82da19ce

mentioned in commit 976400c1

mentioned in commit tnir/gitlab-ce@ec1c71d8