Several visual improvements.
Improved logo display in slim skins.
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.
Merge remote-tracking branch 'upstream/master' into otrs-master