Decouple webide FE from backend compiler API

Assuming we implement #1081 (closed);

the ligo webide will be hosted on netlify. Netlify proxies call to the backend compiler API.

Change the backend compiler API so that multiple compilers are hosted by version in their slug;

/api/${ligo_version}/

Make a json file located at: /api/available_compilers

That lists the available compilers.

The ligo webide will use /api/available_compilers to discover what compilers are available and use the latest stable compiler. The user can choose which compiler version they wish to use.