Add Twitch, error handling, revokation changes, etc

Refreshable connections, refactoring, access-token endpoint ...

- Aded /users/@me/connections/:connection_name/:connection_id/access-token
- Replaced `access_token` property on ConnectedAccount with `token_data` object for refreshing tokens
- Made a common interface for connection things like ComonOAuthTokenResponse
- Added `RefreshableConnection` class
- Added token refresh to Spotify connection (disabled)

add epic games connection

`handleCallback` returns connection if created for `USER_CONNECTIONS_UPDATE`

adding connection now works

add connections