Revamp and speed up user interface

• disables the 6-12 gram options
• enables indefinite caching
• swaps the loading bar for a spinning icon
• reduces the number of clusters that can be retrieved in one go to 3000
• makes clusters and keywords results clickable (this leads to a concordance,
  which in turn is clickable to lead to the text)

Highlight search term in chapter

• chapter view now highlights the search term
• removes jquery js to highlight the term (now it is done in the backend)
• adds initial settings management
• adds more tests

Concordance design and initial chapter view

• updates text displayed when concordance results table has no entries to '0 entries'
• refactors concordance view
• adds functional tests and unit tests
• starts working with jinja templates
• cleans up files
• initial, raw version of a chapter view (when clicking on a concordance line, the chapter is displayed)

Speed up of the concordance

• adds total count of occurrences in concordance
• speeds up the concordance (for instance, by optimising xpath queries)
• fixes issues with non-alphabetic character rendering in the frontend concordance