Implement custom regular expression for external issue tracking.

[AlexanderBeyn]

This PR adds regex as a third style of external issue tracking. The regular expression must have at least one captured group, which will be used as the index for the link. The entire match replaced by the link when rendered.

Fixes #3468

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9ce4d89). Click here to learn what that means.
The diff coverage is 82.05%.

@@            Coverage Diff            @@
##             master    #4265   +/-   ##
=========================================
  Coverage          ?   41.54%           
=========================================
  Files             ?      449           
  Lines             ?    61329           
  Branches          ?        0           
=========================================
  Hits              ?    25480           
  Misses            ?    32496           
  Partials          ?     3353

Impacted Files	Coverage Δ
models/repo_unit.go	64.91% <ø> (ø)
modules/auth/repo_form.go	41.73% <ø> (ø)
routers/repo/setting.go	9.28% <0%> (ø)
models/repo.go	48.39% <100%> (ø)
modules/markup/html.go	80.76% <90.32%> (ø)

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 months. Thank you for your contributions.

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[techknowlogick]

I've resolved the merge conflicts, however due to some recent changes the tests are failing

[lunny]

Please resolve the conflicts

[strk]

@AlexanderBeyn thanks a lot for this, are you planning to resolve the conflicts ?

[strk]

@lunny I've resolved the conflicts in #17624 (using a rebase operation - don't know if I'm authorized of committing to this branch)

[wxiaoguang]

Thank you for your contribution! There is a new PR based on your work.

• Custom regexp external issues #17624