Changes of i18next config

[zoran995]

What this PR does

This is just a draft of some updates for i18next configuration, but it would be nice to discuss them.
This introduces

• Ability for the user to define which languages should be used and fallback language in TerriaMap config file (if not defined default values used). This should replace the instance of i18n.js file (used in prerequisites).
• Introduces usage of i18next-xhr-backend so language files get pulled from the backend (this will require exposing Languages so a browser can access them). More info about it here.
• Introduces dev-dependency eslint-plugin-i18next. This should wait for changes from this pull request. This would be useful to detect hard-coded strings. Needs a lot of work to separate all strings that should not be detected as hard-coded (currently detects over 1k of problems). Currently works only on .js and .jsx files, to make it work for .ts files Model architecture: Enable eslint for .ts files #3303 is needed.

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've updated CHANGES.md with what I changed.

[soyarsauce]

Just a quick note - I had a quick go at implementing a super lightweight usage of something similar to this PR, and saw i18next-xhr-backend is deprecated - i18next-http-backend (https://github.com/i18next/i18next-http-backend) is the favoured lib now, except it is still early days for it as I ran into some problems with using the options as they are developed out.

[zoran995]

loading translations override from backend is moved to #4324, with the usage of i18next-http-backend

[zoran995]

I think this is suppersed by #4519, while other parts are already implemented