Enhancement: Site account extensions

This PR heavily extends the site accounts service:
* Extended the accounts information (not just email and name)
* Accounts now have a password
* Users can now "log in" to their accounts and edit it
* Ability to grant access to the GOCDB

Furthermore, these accounts can now be used to authenticate for logging in to our customized GOCDB. More use cases for these accounts are also planned.

https://github.com/cs3org/reva/pull/2008
