Add 'mapMaybe' function to PrefixTree.
Add remove and difference function to Occurrences/Compressed.

Difference function works with a set of keys using a fold.
Add 'deleteDocs' function to HolIndex and Inverted.PrefixMem.
Add 'deleteDocById', 'deleteDocByURI' and implemented 'updateDoc'

- Add /document/delete/ route (for testing purposes)
- "page not found" is now displayed for unknown routes