Change: Improve UI/UX of collaborator forms

Applied several UI/UX improvements to the collaborator forms (adding and editing).
- Showing avatars for selected collaborators on a new share and fixed styling/layouting of said collaborators in the list.
- Added sensible margins on text about missing permissions for re-sharing in the sharing sidebar.
- Fixed alignment of displayed collaborator in editing view for collaborators.
- Removed separators from the forms that were cluttering the view.
- Moved role description on role selection (links and collaborators) into the form element. Not shown below the form element anymore.

https://github.com/owncloud/web/issues/1186
