User administration page

[warcode]

A page similar to the "Settings" pages, visible only to administrators.

• List all users
• Disable/Ban/Delete users
• Edit users
• Promote users to administrators
• Invite users / Generate invitation links
• Manually approve users after registration

Many of these are already issues as individual items.

[marceloschmidt]

Still lacking invitation as per #194. This is closed now.