Implemented "Creator" and "Contributor" edge, introduced Migration4.

Had to move both new assoc_types from Migration2 to Migration4 to deploy them on our current database. That is because I swapped the contents of our migrations during latest development. Migration2 is now skipped and deployments from scratch should not be affected by this step.

Pending:
- current layout currently does explicitly not support IE9, IE10 and Opera
- mathjax-formula preview