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
Lunny Xiao
David Svantesson