Authorisation

Depends on #22
### Github authorisation strategy
Use "github authorisation strategy" for the REST API and the web interface.

Implementation:
- Token based for REST API #3
- Web flow based for Web interface #2

**1: Check what is "my" username:**
`curl -u 8cbc5851f0a8683c0c0a682f0d8e4380ccf843f:x-oauth-basic https://api.github.com/user`

**2: Check membership of organization:**
`curl -u 8cbc5851f0a8683c0c0a682f0d8e4380ccf843f:x-oauth-basic https://api.github.com/orgs/cargomedia/members/njam`

Docu: https://developer.github.com/v3/orgs/members/#check-membership


### Websocket
Issue our own token by an API endpoint.
Then send this along when connecting to the Websocket server.

See #10
rename to `isUserMember` or pass organization as 2nd variable.