The four existing jointypes, 'open', 'controlled', 'request' and
'invite' are mutually exclusive, but they don't need to be so strict.
This patch introduces more flexibility in the way groups allow new
members to join.

* Group admins can always send membership invitations to a group, even
  if it's open or controlled
* Membership requests can be enabled for any group unless it has open
  membership.
* The grouptype now determines the set of roles available to a group,
  but no longer restricts the available join types.

The db upgrade will preserve existing behaviour apart from enabling
invitations on open, request, and controlled groups.

Change-Id: I8bb0940a37f3c0c36366c1d5b8d27e8b9914a7e3
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>