[enhancement] Improve highlighting

[david-allison]

Highlighting likely won't be perfect, but we can make it a lot better:

• Add highlighting to the main page
• Obtain the positions of the token that match in Lucene
  • See if we can match these to the 'real text' which is displayed to the user - likely possible in 98% of cases - word at index 5 in Lucene will likely be word 5 in the real text.
  • Question: Server-side or client side? Both are equally as powerful, and it seems I'm avoiding Client-side because I'm avoiding JavaScript
• Translations - likely going to be harder, main aim is to fix punctuation

[david-allison]

ref: https://github.com/david-allison/manx-corpus-search/commit/b1d8e0b860cff2cd5fbab8e8830c6a6e2f4e2c41

We'll want unit tests at the very least. I suspect we'll only go with 'fuzzy' highlighting rather than trying to replicate Lucene's tokenisation.