Add client credentials flow

[xerbalind]

https://www.oauth.com/oauth2-servers/access-tokens/client-credentials/

The Client Credentials grant is used when applications request an access token to access their own resources, not on behalf of a user.

A client can get an access token when providing the client id and secret during the oauth flow. I assume that the client secret is only known by the client/application itself.

The client information endpoint is /current_client and only works with a client session.

Currently the only endpoint supporting a client session is the /mails/new and is also only authorized by clients with the mailer role. (added role support for clients)

Tests will follow.

[xerbalind]

I also refactored the different FromRequest implementations for each session type, so it forwards to the next FromRequest when encountering a wrong Session instead of erroring.

[xerbalind]

The distinction between clients having roles (like with users) and roles only applying to specific clients might now be confusing, so I'm open for suggestions :).