set up `api.eessi.io`

[boegel]

We should be using https://api.eessi.io, which allows us to move the API service elsewhere should there be a need for it (GitLab for example).

[boegel]

The JSON data that's currently available via https://www.eessi.io/api_data/data/eessi_api_metadata_software.json should probably be served by a clean URL like https://api.eessi.io/json/software (and then similar for R extensions like https://api.eessi.io/json/R-extensions, etc.)