CLC-2939 - Add validation / errors to user search in "Add People" tool

https://jira.ets.berkeley.edu/jira/browse/CLC-2939

- Adds input validation on requests to GET /api/academics/canvas/course_add_user/search_users.json, with 400 error if 'search_text' is blank, or 'search_type' parameter is invalid.
- Updates search form to change type on search text field from 'text' to 'number' when 'Student ID' or 'CalNet UID' search type is selected.