* keep sure if assigneeIDs == nil -> do nothing

* fix #8872

* Revert "keep sure if assigneeIDs == nil -> do nothing"

-> go handle it itself preaty well

This reverts commit e72d94129c.

* clarity comparson

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* simplify

* Update models/issue_assignees.go

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* Update issue_assignees.go

* simplify more

* add --if oneAssignee != ""-- again

* Update models/issue_assignees.go

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* CI.restart()

* Update issue_assignees.go

* add Test for GetUserIDsByNames

* add Test for MakeIDsFromAPIAssigneesToAdd

* fix test

* Move more issue assignee code from models to issue service

* fix test

* 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

* refactor: append, build variable and type switch

* fix: remove redundant space.