Fix safari edit mode and entity preview

This merge should resolve #156 and #160

The general strategy here was fixing as much unit tests as possible for Safari and then hope that the edit mode (and entity preview) is fixed automatically.

Therefore, a lot of small changes have happened. However, the most important take-away is that Safari is way more strict (or dumb?) when it comes to XML, XSLT and HTML. E.g. it does not allow <tr> elements without direct <table> parents, it is very pedantic with XML namespaces (conversion from 'XMLDocuments to HTMLDocuments and vice-versa can break things). Apparently, DocumentFragment(a lightweight alternative toDocument`s) could not be transformed by our XSLT functions - I still don't understand why.

Note: I also removed the (unnecessary) ext_revisions module because the tests kept failing in safari and I didn't want to deal with them.