Save user language setting to backend

[awarecan]

fixes: #2740

• Save language selection to backend's "frontend per user storage"

• Still save/load selected language from local app storage, but the backend storage will override local storage

• Use local app stored language or browser language setting before login (might add language selection in login form, out of this PR's scope)

• Add support for zh-MY and zh-MO locales

• Convert themes-mixin.js to TypeScript

• Convert ha-pref-storage.js to TypeScript

[balloob]

I think it would make most sense if we treat the backend storage as a sync, but don't integrate it with the translations startup code. That will keep the app faster to load and keep the code cleaner.

• Keep storing language pref in local storage and we'll load from that on page load
• On page load, query the backend make sure the language is up to date, and call setLanguage if it is not. Do this call async, not blocking further startup. A one time sync post-login to set the language is totally fine.
• Set language will behave normally, storing it in local storage. Will also send request to the backend.

[awarecan]

Address some review comment, all functions worked as expected excepted no local app storage yet.

I am still want to refactor the whole translation/language code base to make the flow more clearly to follow balloob's suggestion. More commits are coming

[balloob]

ok to merge when final comments addressed 🎉