login system for streamers

[strager]

After a streamer logs in, we should recognize that they are logged in in the app.

This feature unblocks settings (#18, #17).

• [x] Rename AuthDb -> TwitchAuthDb
• [x] Rename UserId -> TwitchUserId, Token -> TwitchToken, etc.
• [x] Rename UsersDb -> TwitchUsersDb
• [x] Create FirstUserDb (id | twitch_user_id | app_enabled | first_reward_id | created_at | updated_at)
• [x] Web server creates or updates user on login (FirstUserDb.create_or_get_user(twitch_user_id: TwitchUserId) -> FirstUserId)
• [x] Web server manages currently-logged-in user
  • [x] log out button
  • [x] current user indicator in header