`Auth`: implement endpoints

[Seerden]

See #2.

Changes:

• implement the following endpoints with their respective request handlers:
  • [x] POST /user/login
  • [x] POST /user/logout
  • [x] GET /user/me

    Miscallaneous:

• [x] initialize redis connection on startup (copy-pasted from seerden/mana)
• [x] install express-session
• [x] extend express Session and Request types with user properties

[Seerden]

Manually tested the endpoints through Postman. Still have to pick an end-to-end testing solution.