introducing documents with history

fixes #3 
* provides documents in their own app. They can be used as protocols, information pages... you name it
* provides a markdown editor that creates revisions of each change to a document
* provides a version diff viewer that show differences for each version and also lets the user revert a document to an earlier version without loosing that version
* is tested sufficiently
* editor and diffviewer are reusable --> see information_pages app for usage

--> it is not possible to create a new information page right now but I think this is work for the future. This sets the base for further work with information pages and protocols

in a migration?