Add static metadata output on project save

* Uses project translations to build supported language list
* Saves to metadata.json on project save/build, uploads to app servers
Add embedded perl module for nginx subdomain redirects

* Add ReadTheDocs.pm for reading metadata file as json
* Add tests for RTD.pm
* Update nginx salt config
* Uses project language settings for nginx redirect
Merge pull request #690 from agjohnson/subdomain-redirect-home

Redirect root domain URLs for docs to default version...without hitting Django app