Sponsor membership should now be managed through a POST request

* property DAV: group_member_set can be used for setting (accepted) members, but
* to add a membership request you need a POST request
* to promote or demote users to admin, you need a POST request

Privileges aren't checked yet!
Merge remote branch 'origin/pieterb_conext' into niek_vanalles
UNTESTED: made it possible to change group memberships through a POST request on the group
Merge remote branch 'origin/pieterb_conext' into niek_vanalles

Conflicts:
	src/beehub_group.php
	src/beehub_sponsor.php
	views/sponsor.php
UNSTABLE: Fixed the sponsor part largely
Merge remote branch 'origin/pieterb_conext' into niek_vanalles

Conflicts:
	src/beehub_group.php
	src/beehub_sponsor.php
Improved bootstrap, with proper style resetting.
Merge remote branch 'origin/niek_vanalles' into pieterb_conext