Chatkit setup

[chakeresa]

Changes proposed in this pull request: Alter the authenticateUser mutation:

• If the user doesn't already have a corresponding chatkit user, create one
• Generate a JWT token for the user
• Return the JWT token so that it can be used for later authenticated requests (to chatkit and CrowdHound)

Adds the pusher-chatkit-server gem, as well as ChatkitService.

Resolves #94

The following checks have been completed:

• [x] Tested my new feature(s) as well as any feasible edge cases (if possible)
• [x] Checked coverage/index.html - did not add any new code that's not covered by testing (if possible)
• [x] Merged in the latest master to my branch with git pull origin master & resolved merge conflicts
• [x] Ran rails db:migrate
• [x] Ran the test suite - all tests are passing (or maybe skipped)
• [x] Checked affected endpoints in Postman / GraphiQL
• [x] Updated README for changes (new endpoints, new gems, etc)