[#79323508] Remove assignment manager
[#79323508] Remove assignees from task
remove assignment mailer
WIP [ci skip]
Merge branch 'master' into assignments-b-gone

Conflicts:
	app/assets/javascripts/templates/layouts/assignee_overlay_layout.hbs
[#79323508] PR fixes
Merge branch 'assignments-b-gone' of github.com:Tahi-project/tahi into assignments-b-gone
[#79323508] Remove unnecessary task#assignee overrides
[#79323508] Removal of redundant test; make actual test more clear
Merge branch 'assignments-b-gone' of github.com:Tahi-project/tahi into assignments-b-gone
Let prior task assignees fall on the floor
Remove extraneous assignee_id test data
Blank overlay is still needed.  Readd it.
Make journal_reviewers possible_reviewers.
Add clarifying comments to model.
PaperAdminTask has possibleAdmins.
Get rid of assignees.

Remove an extra available_admins use too
Editors becomes possibleEditors
Remove unused editor field on paper.js
No longer a task owner