remove systems from groups using POST, with confirmation

Bug: 970512
Change-Id: I33fecf5b2c0de500186cea803a390c765f19573b
rationalise group pages

/groups/admin is redundant, use /groups/ instead.

/groups/group_members is useless, use /groups/edit instead (with UI
adjusted depending on permissions).

Bug: 970493
Change-Id: Iebb24aefb19b73f5c9bc9ecbdb983572759ceed2
allow only admins to add systems on the group page

Group owners shouldn't be able to add arbitrary systems to their group,
since that lets them gain extra access to the systems. This patch
adjusts the permission checks, and also hides the form for adding
systems on the group page from everyone except admins.

System owners can still add their system to a group from the system
page.

Bug: 970499
Change-Id: I85472550068265e3feb544064c3b54af2b91a436
Merge changes I85472550,Iebb24aef,I33fecf5b into develop

* changes:
  allow only admins to add systems on the group page
  rationalise group pages
  remove systems from groups using POST, with confirmation