load translation override using backend

[zoran995]

What this PR does

I decided to break commit #4227 into smaller parts since something like eslint will take a long time to completely configure. The basic idea is described in #4280 (comment)
Adds an option for overriding the translation files from TerriaMap. This provides the user with the ability to

• define language configuration inside TerriaMap config file.
• put a language overrides to file wwwroot/Language/{{lng}}/languageOverrides.json, {{lng}} is a short name for the language. If the language that user is looking for is not officially supported by Terria, he should specify also create a translation.json to avoid 404 not found error in console. Reason for this is in i18next backend default behaviour to try to load every namespace that is not already loaded.
• tests are updated and should be correct (at least they are on my computer) to support the removing of i18.js from prerequisites.js. Tests now use cimode language option which returns the translation key, it is defined to run beforeEach test so it is not necessary to call it for all tests.

global internationalization track in #3785

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist) - I am not sure how to test this
• I've updated CHANGES.md with what I changed.

[zoran995]

One idea is to use this as an option to enable internationalization of the help and trainer content (if it proves to be possible to handle all use cases hope to have it by end of the week)

[soyarsauce]

This should go some of the way to addressing https://github.com/TerriaJS/nsw-digital-twin/issues/425

[zoran995]

implemented in #4523