Added FileList.setViewerMode to hide controls

Some files app embed themselves under the controls (like the text
editor). The new method FileList.setViewerMode() makes it possible to
properly show/hide the control buttons using the correct permissions.

Apps using this approach must call setViewerMode(true) when starting and
setViewerMode(false) upon closing to restore the controls.

This is needed for #5284
Added warning notification when user home already exists

When creating a user and the home already exists in the data dir, a
warning notification will be displayed.

Fixes #5161
Merge pull request #5476 from owncloud/admin-showwarningwhenhomeexists

Added warning notification when user home already exists
Merge pull request #5473 from owncloud/files-startviewerfunc

Added FileList.setViewerMode to hide controls
Merge branch 'master' into prevent_user_from_creating_or_renaming_to_an_existing_filename

Conflicts:
	apps/files/js/filelist.js