Registration security

Fixed security problem with authentication. Token is now generated on server instead of in the app. Therefore also the username must be sent to the server during registry, login and upload. This should be merged soon because all web tests are currently failing (besides the other tests..). After that, the changes on the web server will also be deployed on catroid.org.

Jenkins testruns:
JUnit:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test-source-and-jUnit-tests-only/51/

Web package:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-package-test/65/
--> testLoginWhenUploading() is failing because it is not possible to distinguish between the same usernames in upper- and lowercase anymore due to changes on the web server (I guess because username is needed for unique authentication, which is converted to lowercase on the web server (@chrisss404 ?). But I'll have a look at it.