Integrate slack-dm-link endpoint

[Kironb]

Since we want to integrate LCS slack-dm-link endpoint with mentorq, we could potentially do this from the backend. This may also be a frontend issue.

[Anarav]

Requirements discussed with front-end:

• [ ] Make contact field not required (as long as slack-id is set within their LCS profile)
• [x] Create the mentorq slack-dm endpoint as an extension of a ticket (so something like ticket//contact)
• [ ] Properly return the HTTP response code and message (if there is LCS error)

[Kironb]

• [x] Pass in whether or not the user has a SlackID during initial login