Employee signin and signup

Tasks:
- [x] Add users table migration

- [x] Implement /login endpoint as per the [flow](https://github.com/joshsoftware/peerly/wiki/API-side-Login-and-Signup-flow)

- [x] Add user_blacklisted_tokens table

- [x] Add logout API (this should create an entry of user token in user_blacklisted_tokens)

- [x] Task to clean up user_black_listed token when the token in table expires expires

- [x] Implement a way to check for valid login/JWT on each request (something we can call in http handler functions)

Acceptance criteria:
1. New / Existing users should be able to login
2. All users signed up from this flow should have `Employee` role