Add "Add People to Group" button on ad-hoc group user roster pages

This PR adds the "Add People to Group" button on ad-hoc group user roster pages. There are two components:

- an addition to the SFU API that renders the HTML for the button if:
  - the group is a member of the "Ad-Hoc Groups' group set
  - the current user is the group leader
- an addition to sfu.js that calls that endpoint, and if it gets back a successful response, inserts the button into the DOM and adds an event handler for clicks (which opens the add-user tool in an iFrame)