Reference issues from pull requests and other issues (#8137)

* Update ref comment

* Generate comment on simple ref

* Make fmt + remove unneeded repo load

* Add TODO comments

* Add ref-check in issue creation; re-arrange template

* Make unit tests pass; rearrange code

* Make fmt

* Filter out xref comment if user can't see the referencing issue

* Add TODOs

* Add cross reference

* Rearrange code; add cross-repository references

* Striketrhough obsolete references

* Remove unnecesary TODO

* Add "not supported" note

* Support for edits and deletes, and issue title

* Revert changes to go.mod

* Fix fmt

* Add support for xref from API

* Add first integration test

* Add integration tests

* Correct formatting

* Fix add comment test

* Add migration

* Remove outdated comments; fix typo

* Some code refactoring and rearranging

* Rename findCrossReferences to createCrossReferences

* Delete xrefs when repository is deleted

* Corrections as suggested by @lafriks

* Prepare for merge

* Fix log for errors

options/locale/locale_en-US.ini

@@ -836,6 +836,10 @@ issues.create_comment = Comment
 issues.closed_at = `closed <a id="%[1]s" href="#%[1]s">%[2]s</a>`
 issues.reopened_at = `reopened <a id="%[1]s" href="#%[1]s">%[2]s</a>`
 issues.commit_ref_at = `referenced this issue from a commit <a id="%[1]s" href="#%[1]s">%[2]s</a>`
+issues.ref_issue_at = `referenced this issue %[1]s`
+issues.ref_pull_at = `referenced this pull request %[1]s`
+issues.ref_issue_ext_at = `referenced this issue from %[1]s %[2]s`
+issues.ref_pull_ext_at = `referenced this pull request from %[1]s %[2]s`
 issues.poster = Poster
 issues.collaborator = Collaborator
 issues.owner = Owner