Fix insertion id handling in TreebankRetriever

The changes made in #451 also need action in the Retriever to work
properly!
More carefulness when persisting morph info

We don't want to add 'undefined' as a string - this can especially
happen with artificialTokens
Merge pull request #452 from latin-language-toolkit/at_fixes2

Temporarily fix for faulty artificialToken handling on a Perseids Backend II
Minor refactor in saver
Add custom build of highlightjs
Collect persisters with output fn in saver
Add output fn to TreebankPersister
Start with outputter directive
Add outputter to navbar buttons
Add scss for a large modal
Add an XML formatting function to arethusaUtil
Make highlight css files importable for scss
Fix scss for outputter modal
Allow to xml in TreebankPersister.output()
Add angular-highlightjs directive
Update outputter directive
Update arethusaUtil.formatXml()

Write empty nodes as one liners. `<word/>` instead of `<word>\n</word>`
Fix font-weight of highlightjs code
Update scss for outputter
Make outputter directive functional