Implemented basic support for permissions.
Implemented permission check for timesheet and expense extension.
Code cleanup.
Fixed installation of permissions for expense extension.
Errors when adding users are shown correctly.
Improved logged permissions error messages.
Fixed updater to set admin group membership correctly.
Added missing function to get groups of an activity.
Fixed several bugs.
Membership of users in groups can be set with a membership role.
Merge branch 'permissions'