CLC-3622 - Resolve intermittent issue with 'Add People' tool link

https://jira.ets.berkeley.edu/jira/browse/CLC-3622

Replaces logic for adding 'Add People' tool link in the 'People' section of a course site. 

* Uses the new External Tools API (/api/academics/canvas/external_tools.json) to obtain the ID of the 'Add People' tool, rather than obtaining the link from the course navigation menu.
* Continuously loops on 'People' pages for a course only until the hidden 'Add People' button is found. Once found it stops the loop and adds the link after the hidden button.