Continuing the New User management

:construction: PR just for collaborating. Work In Progress, don't click merge yet.

## First priority, basic functionality
 
 * [x] the settings area should _slide_ out smoothly, like in News. Also it should be possible to close it again by clicking on the same header area. And the header shouldn’t have a different color than the drawer. Best just try to extract it out from News and transfer the styles to core.
 * [x] Search filter needs to rely on server response, because not necessarily all users are already in the list (by @blizzz )
 * [ ] the undo notification should disappear after 7 seconds and perform the the delete operation (by @DeepDiver1975 / @jancborchardt )
 * [x] creating a group on the drop down menu dies not update the group list on the left (by @DeepDiver1975 )
 * [x] creating a group on the group panel does not update the list (by @DeepDiver1975 )
 * [x] delete of group does not trigger delete on the server (by @DeepDiver1975 )
 * [x] adding a group should not be an input and button right away, but rather work like adding Groups in the Contacts app (inserting a new entry below) (by @jancborchardt)
 * [x] the clickable area of the group actions (rename, delete) needs to be the whole row height and width of 44px
 * [x] the rename button should be directly next to the name, just like in the Files app
 * [x] the rename and delete icons should show half-transparent, and only dark on hover – maybe only use the .action class, just like Files and News do
 * [x] create users fails with: Fatal error: Call to undefined method OCP\JSON::checkSubAdminUser() in /home/blizzz/owncloud/dev/master/settings/ajax/createuser.php on line 4 (by @blizzz)
 * [x] filtering users oftentimes does not work, filtering admins almost never works (shows more people) (by @jancborchardt )
 * [x] we need a spinner as feedback when switching groups (by @jancborchardt )
 * [x] when a group is selected/shown, mark the entry in the sidebar as active. Just like in Notes. (by @jancborchardt )
 * [x] The delete group action should only show up for the currently active group. That is, _not_ on hover. (by @jancborchardt )
 * [x] The »Create« buttons have too big of a font-size (18.33 px, cause of 0.8 em of the body) (by @jancborchardt )
 * [x] »admin« is a set group, it shouldn’t be possible to delete (right @DeepDiver1975?) (by @jancborchardt )
 * [x] In addition to deletion, we need a »rename« action on the group (by @jancborchardt )
 * [x] We need search in form of a filter (by @jancborchardt )
 * [x] selecting a group on the group panel does not update the user list (by @DeepDiver1975 )
 * [x] rename Storage to Quota (by @blizzz)
 * [x] groups should be sorted by number of people in them, not by alphabet (by @jancborchardt)
 * [x] groups with no members should have no number, instead of a 0 (by @jancborchardt)
 * [x] the sidebar should be position: fixed so it doesn’t move around when scrolling (should be in the core styles) (by @jancborchardt)
 * [x] the app-navigation needs a higher z-index than the content, so that the content does not overlap the sidebar when you scroll horizontally (should be in the core styles, by @jancborchardt)
 * [x] the »Create« button should say »Add user« to be more specific (by @jancborchardt)
 * [x] the settings area should be a slide-out drawer like in the News app (by @jancborchardt)
 * [ ] What shall happen when clicking on a filtered group? – Show all users of that group, but show those first which also match the filter term. Maybe with a divider below. (by @blizzz )
 * [x] Click on Everybody: No, it should display »Everybody« but filtered by the filter. Only resetting the filter should reset the filter. (by @blizzz )
 * [x] To reset filter: »clickable x icon in the filter box to empty the filter«(by @blizzz )

## Additional features

 * [ ] should be usable to manage huge amount of users (>20k) (by @blizzz )
 * [ ] pre-defined filters: Everyone, Users by Backend (Internal, LDAP, etc.), Not Grouped (users without group), Disabled Users. They shall be shown only if they contain rows, and be post-loaded using Ajax (by @blizzz )
 * [x] Last Login requires core to store last login/session refresh (by @blizzz )
 * [ ] column for email (after Full Name), lets admin edit the email (by @blizzz )
 * [ ] optional columns (to activate by checkbox on the bottom left):  Usage (displays Quota usage in %, after Quota), followed by Storage Location, Last Login (last col). Choice of which are displayed should be remembered  (by @blizzz )
 * [ ] checkbox to send email to users after creation of the account (by @blizzz )
 * [ ] when encryption app is enabled and a recovery key is available: on password change ask for recovery key by prompt so that files are recovered. see current implementation as example on how it works. (by @blizzz  \cc @schiesbn )

## Big features which have to wait first

 * [ ] multiselect and edit of users (as in Files or Contacts-master app) (by @blizzz )
 * [ ] option to disable internal users (by @blizzz )
 * [ ] sortable columns (by @blizzz )