Improve the robustness of authinstance-in-session handling, to handle an unduplicatable bug (at least by me).

All users who log in to the system should have a valid authinstance in their session. Apparently however, some don't. I can't narrow down how this happens, but this patch will make sure things don't blow up in this case.

Conflicts:

	htdocs/view/columns.php
	htdocs/view/layout.php

When assigning view access, allow users to assign access to any group, not just the ones they're in.

'Owner' is a hangover from the old group stuff anyway. It doesn't make sense to delete a group just because a group admin has been deleted.

Add two more functions - group_param_{group,user}id, which replace a whole lot of duplicated validation in lib/group.php.

Previx get_grouptype_options function with group_. Added some miscellaneous comments to describe how lib/group.php is ordered.

Perform validation on the parameters while we're in the area.

May as well have one, given we have group_delete.

Takes a data structure that describes the new group, and checks that the new group will be created in a sane way.

Replace delete_group with a new function, group_delete. Same implementation, but has a doc comment and parameter checking.

Add doc comment to group_user_can_edit_views. Correct error messages for the InvalidArgumentExceptions.