Mail assignee when issue/pull request is assigned (#8546)

* Send email to assigned user

* Only send mail if enabled

* Mail also when assigned through API

* Need to refactor functions from models to issue service

* Refer to issue index rather than ID

* Disable email notifications completly at initalization if global disable

* Check of user enbled mail shall be in mail notification function only

* Initialize notifications from routers init function.

* Use the assigned comment when sending assigned mail

* Refactor so that assignees always added as separate step when new issue/pr.

* Check error from AddAssignees

* Check if user can be assiged to issue or pull request

* Missing return

* Refactor of CanBeAssigned check.

CanBeAssigned shall have same check as UI.

* Clarify function names (toggle rather than update/change), and clean up.

* Fix review comments.

* Flash error if assignees was not added when creating issue/pr

* Generate error if assignee users doesn't exist

options/locale/locale_en-US.ini

@@ -977,6 +977,7 @@ issues.review.review = Review
 issues.review.reviewers = Reviewers
 issues.review.show_outdated = Show outdated
 issues.review.hide_outdated = Hide outdated
+issues.assignee.error = Not all assignees was added due to an unexpected error.
 
 pulls.desc = Enable pull requests and code reviews.
 pulls.new = New Pull Request