Config: Extract languages to shared config and remove obsolete muse_supported_themes

[gziolo]

We repeat across all config files so many values. We could easily mitigate that issue by using already present _shared.json config file introduced by @mjangda in #876. This PR tries to slim down config files by extracting muse_supported_themes and languages fields. We probably can open the follow up PR with more changes.

Testing

1. Run npm test and make sure everything passes.
2. Run http://calypso.localhost:3000/me/account and make sure you still can change language for your interface :)
3. Go to your site's setting page and check if you can change language of your site.

[gziolo]

/cc @mtias @rralian @mjangda @artpi

[gziolo]

Can someone double check if that works properly for users? Everything else displays properly for me, but this one seems to be broken.

[mtias]

I haven't tested the language ones, but it's looking good to me.

[gziolo]

It turned out muse_supported_themes value is no longer referenced in code. I removed it. @sirbrillig can you confirm that change?

@akirk can you double check language related change?

[sirbrillig]

@gziolo wow, I did some digging and the use-case for muse_supported_themes was removed on August 4, 2015. (It had been added on Mar 4, 2015 to prevent showing any customize link in the sidebar (client/my-sites/sidebar/sidebar.jsx) when Muse would not be available, because the native customizer experience at mobile width was so poor.)

I'd say removing it is fine.

[gziolo]

@mtias can we land this one? It looks like there are no blockers.

[mtias]

Yeah, let's 🚢