[paradigm dictionary] paradigm dictionary mode (metaissue)

A bilingual dictionary search/lookup with paradigm displays (per [Paradigmatrix](https://apertium.github.io/apertium-paradigmatrix/)).

(Might supersede #105, although that might be envisioned as something a little different.)

* [x] Basic UI functionality
* [x] Display search results from APy `bilsearch` mode
* [x] Bilingual searching and morphological mode searching
* [x] Add something like [@unhammer's suggestion in #105](https://github.com/apertium/apertium-html-tools/issues/105#issuecomment-835856793). (added for uum-eng in https://github.com/apertium/apertium-uum-eng/commit/aaa0a0ecfdfcb2d10bc2c247b99af06a35fe35bc)
* [x] Add APy mode for `billookup` (https://github.com/apertium/apertium-apy/pull/251)
* [x] Display search results from APy `billookup` modes
* [x] paradigm displays
* [ ] "Fuzzy" search options
* [ ] Substring (via front-end option passed to APy `billookup`, which perhaps uses different mode in translation pair)
* [x] Semantic (via word embeddings implemented as weighted monolingual transducer)
* [ ] Spelling (via libvoikko model?)
* [ ] backend miscellanea
* [ ] support paradigm definitions that are HTML tables with space holders (as in paradigmatrix)
* [ ] labels to localisation files (https://github.com/apertium/apertium-html-tools/issues/517)
* [ ] [remove code from dictionary mode paradigm definition files](https://github.com/apertium/apertium-html-tools/issues/520)
* [ ] frontend fixes
* [ ] synchretic forms fixes (https://github.com/apertium/apertium-html-tools/issues/521)
* [ ] UI enhancements (cf. "style preferences")
* [ ] Customisations (e.g., different orthographic modes)
* [ ] Dialects? (probably same way as in translation and analysis/generation)
* [ ] Ability to turn off various "fuzzy" search options (perhaps separate from the language-specific stuff?)
* [ ] Advanced features
* [ ] Segmentation / morphological explanations
* [ ] Pivot (multi-step) lookup?
* [ ] Multi-language results?
* [ ] audio recordings?
* [ ] texts?
* [ ] Documentation (beginnings here: https://wiki.apertium.org/wiki/Apertium-html-tools/Paradigm_dictionary)
* [x] Basic description of features
* [x] Basic installation instructions
* [ ] Fuller description of features and how it works
* [ ] Fuller installation instructions