The link property of Facebook Oauth profiles no longer has a consistent
value across logins and will be completly removed in future versions of
Facebook's Graph API.

We generate a link in the old format from the id for backwards
compatibility.

This improves/fixes compatibility with third-party OAuth APIs.

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

 - add controller/service/entity/dao-functions
 - add motdlist so that users can tell the backend, which motds they have read
 - add admin account (in arsnova.properties: security.adminaccount).
 - add permission function for admin

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.

The API returned "ldap" instead of "guest" for guest authentication
before.

Renamed TEACHER to SPEAKER to adapt roles to ARSnova client, and
also added some convenience methods for role handling.

add type of user account
adjust test for new user account type
UserServiceTest which test persistence of the maps

remove websocketcontroller and let sessioncontroller do the
authorization
add toString to User entity

* Make guest login working with new UserService
* Reuse existing guest username
* Adapted tests

switch to oauth2 authentication which allows facebook, twitter, google
etc authentication