HTML view for treebank diffs

Adds an HTML view for treebank diffs. The route would generally also work with translation alignment documents, but this isn't supported yet and will come at a later stage.

Integrates with Perseids by passing `backend=perseids` as param - id's of treebank document can then be given as plain numbers, such as `/treebank/diff/html?gold=4186&reviewable[]=4111&reviewable[]=4110`

The HTML stuff still needs more work and is pretty raw at this point, but probably already a nice thing to have.

Mind that we still aren't playing nice with treebank files that contain non-standard postags - this is an important next step to take with this service.

The `Parser` classes now also take the treebank attributes `format` and `xml:lang`, as well as `annotator` information into account - also handled very primitive at the moment.