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* Remove outdated comments; fix typo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

models/migrations/migrations.go

@@ -244,6 +244,8 @@ var migrations = []Migration{
 	NewMigration("add email notification enabled preference to user", addEmailNotificationEnabledToUser),
 	// v94 -> v95
 	NewMigration("add enable_status_check, status_check_contexts to protected_branch", addStatusCheckColumnsForProtectedBranches),
+	// v95 -> v96
+	NewMigration("add table columns for cross referencing issues", addCrossReferenceColumns),
 }
 
 // Migrate database to current version