Enhancement: Application passwords management

This PR adds the functionality to generate authentication tokens with limited
scope on behalf of registered users. These can be used in third party apps or in
case primary user credentials cannot be submitted to other parties.

https://github.com/cs3org/reva/pull/1719
https://github.com/cs3org/reva/issues/1714
https://github.com/cs3org/cs3apis/pull/127
