Improvements to wordcloud

The wordcloud can be given some extra functionality to be more informative.

- [ ] Add the option for the table to show percentages instead of absolute frequencies
- [ ] If the user has entered a query or filter and is viewing a subset of the corpus, they can currently see the term frequencies in their subset. Also add the option to see frequencies relative to the whole corpus, e.g. as TF-IDF score.
- [ ] Add a setting to show bigrams/trigrams instead of unigrams. Since the wordcloud relies on a CountVectorizer, this should be trivial, though it may be problematic for the TF-IDF mentioned above.