[webui] display all users, sign-up form only in non-proxy mode
[webui] Add tokeninput gem for group adding and editing
[webui] Fix group controller/view syntax
[webui] Require group parameter for group/edit
[api] another attempt to speed up also the request rendering up again.

Empty match arguments are not allowd anymore to avoid people wonder about
timeout errors when every request gets rendered. An explicit match='*' to
achieve the same is still supported.
[api] allow to double-add users to groups , they appear just one time afterwards. (this helps the webui)