29 Add localisation to the front-end

What? Why?

Related to issue #29

This PR adds localisation of strings, so that they are translated based on the URL 'lang' parameter.

Code-specific details (for Reviewer)

I used the react-i18next library to do translation of the UI strings in React components. See the setup in the i18n.ts file. This file actually fetches the dataset config.json itself (separately to configSlice.ts) and then transforms the ui vocab into a format that can be consumed by i18next.

Perhaps it would have been more in line with the rest of the app to share the vocabs state in Redux that is used elsewhere (e.g. for displaying filter input labels), and create our own method to place translations into the UI. But I chose to use i18next since it's a widely-used, standard library and we get a lot of nice things with it for free (e.g. the useTranslation hook, context-dependent string interpolation such as translating plurals to different languages). And in the future, we'll probably want to split out UI translations that are common to all maps from individual dataset vocabs anyway.

Checklist

[ ] Updated or added any necessary docs in docs/
[ ] Added UTs (wasn't sure how to write valuable UTs for this, since most logic is done by external libraries)
[ ] Checked that all automated tests pass

What should we test? (for QA)

Change language with ?lang= URL paramater to a language supported by the dataset (e.g. 'fr' with dataset-A)
Check that all strings in panels, popups, and the basemap are translated
Change lang to a bad string and check that language defaults to 'en'

DigitalCommons / mykomap-monolith