implement API to check Canvas user existence

[lsloan]

Related to #19 and #221.

Given a single user's login_id, return an HTTP status code and data to indicate whether a Canvas account exists for the user.

• Exists: HTTP 200; data including login ID, given name, surname, and email address (additional data may be helpful). (NB: Email address is not available to the admin user token with its current set of permissions, but it is not important.)
• Nonexistent: HTTP 404; data should contain an error message and include the requested login ID for confirmation and debugging purposes.

[pushyamig]

The current tool is showing the first and lastname along with email address. Here is the API response from canvas when you look up user

[{"id":605615,"name":"FacebookCCM4 Mailinator","created_at":"2020-06-25T17:44:32-04:00","sortable_name":"Mailinator, FacebookCCM4","short_name":"FacebookCCM4 Mailinator","sis_user_id":"facebookccm4@mailinator.com","integration_id":"112760453749885@facebook.com","sis_import_id":null,"login_id":"facebookccm4+mailinator.com"}]

So based on these facts then I feel it is ok to show first and lastname. Screenshot for reference.

[lsloan]

Issue description updated.