Added code to remove redundant scheduler registration update tasks.
Fixed: memory cache does not get updated (bug#10820).
Fixed: MyServices select misbehaves on empty values (bug#10828).
Fixed comment.
Translate `Valid' in System Maintenance
Added the possibility to define a custom logo per skin. Fallback will be still AgentLogo.
Added possibility to search for 'NotTicketFlags', thanks to Moritz Lenz.
Only auto-set seen flag if article was created by an agent.
Fixed: Backup script should delete incomplete backups (bug#10665).

Conflicts:
	CHANGES.md
	scripts/backup.pl
Fixed some issues with NeedSpellCheck, thanks to S7.
Follow-up fix for bug#10813: add missing destructor.