Unsubscribe from thread through link in email footer

Closes #3437 (closed)

This would be awesome to have! @dzaporozhets can you review?

@zj I have feeling this merge request has a lot of refactoring that is not related to "Unsubscribe" feature itself. Am I right?

Only parts of the code I had to touch anyway are refactored when needed. For example app/mailers/emails/notes.rb seemed heavily refactored, though its only the removal of a block.

@dzaporozhets Want me to take this over? I've been helping @zj with a lot of MRs lately ("Merge when build succeeds" included).

@DouweM sure. Please do!

@DouweM Just implemented #can_unsubscribe? and rebased against master. Anything else?

Added 904 commits:

• 6b2538be...c81647ae - 902 commits from branch gitlab-org:master
• 58867eff - Unsubscribe from thread through link in email footer
• 36d858bc - Add #can_unsubscribe? to SentNotification

Added 1 commit:

• 26cedc7e - Minor improvements, unsubscribe from email footer

Added 1 commit:

• be084908 - #can_unsubscribe? -> #?unsubscribable?

Done.

Enabled an automatic merge when the build for zj/gitlab-ce@be08490863b76026b8f3ffbc422cb7f5d8b4a6a4 succeeds

mentioned in commit b6ae2def

Status changed to merged

Lol, three days between "Enabled an automatic merge" and "Status changed to merged". Thanks again for merge when build succeeds ;)

mentioned in commit 65aafb99