Move method calls related to execution of moving an issue into separate
private mothods. The reason is that then these methods can be easily
extended with EE specific code
(https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4550#note_59158691).

Related gitlab-ce#41949

Closes #33423

This reverts commit e00fb2bdc2090e9cabeb1eb35a2672a882cc96e9.

# Conflicts:
#	.rubocop.yml
#	.rubocop_todo.yml
#	lib/gitlab/ci/config/entry/global.rb
#	lib/gitlab/ci/config/entry/jobs.rb
#	spec/lib/gitlab/ci/config/entry/factory_spec.rb
#	spec/lib/gitlab/ci/config/entry/global_spec.rb
#	spec/lib/gitlab/ci/config/entry/job_spec.rb
#	spec/lib/gitlab/ci/status/build/factory_spec.rb
#	spec/lib/gitlab/incoming_email_spec.rb

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

Commit 43d8bdb4 introduced additional
checks for permissions to read issue in references extractor.

According to endbosses' suggestions.

[ci skip]

This also refactores ProjectSelect adding some decorator-like functions.

Discussed it here:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2831#note_4190228

This also wrapps entire process into transation, as rewriting references
may have large memory footprint.