Send notifications for mentions in pull request code comments

[jpraet]

Fixes #14187: mention handling extracted from email notification code
Fixes #14013: add notification for mentions in pull request code comments
Fixes #13450: Not receiving any emails with setting "Only Email on Mention"

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@ac88b0e). Click here to learn what that means.
The diff coverage is 48.99%.

@@            Coverage Diff            @@
##             master   #14218   +/-   ##
=========================================
  Coverage          ?   41.98%           
=========================================
  Files             ?      735           
  Lines             ?    78881           
  Branches          ?        0           
=========================================
  Hits              ?    33117           
  Misses            ?    40313           
  Partials          ?     5451           

Impacted Files	Coverage Δ
modules/notification/ui/ui.go	62.96% <12.50%> (ø)
modules/notification/base/null.go	77.77% <40.00%> (ø)
modules/notification/mail/mail.go	38.88% <42.85%> (ø)
services/mailer/mail_issue.go	62.65% <44.44%> (ø)
models/issue.go	57.01% <50.00%> (ø)
services/comments/comments.go	69.23% <50.00%> (ø)
services/issue/issue.go	32.87% <50.00%> (ø)
services/pull/pull.go	42.28% <50.00%> (ø)
services/pull/review.go	50.98% <50.00%> (ø)
services/mailer/mail_comment.go	75.00% <69.56%> (ø)
... and 4 more

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[lunny]

May also resolved #13450

[jpraet]

May also resolved #13450

I tested and it doesn't solve that problem. It is because the visited map[int64]bool is populated even when the notification wasn't sent. So when sending the mail to the mentioned users it thinks an email notification has already been sent. Easiest fix is probably to just switch the order around in mailIssueCommentToParticipants: first send to mentions, then to other participants?

I notice something else: you can explicitly unsubscribe from an issue to no longer receive notifications from it. However, in the case where someone explicitly mentions you, shouldn't we still send a notification?

[lunny]

Unsubcribe should not affect mention. But user has an option about mail notification. It could be all, mentioned and none.

[jpraet]

#13450 should be fixed now as well. And unsubscribe no longer affects mentions.

[6543]

@jpraet pleace send backport :)