Change drupal-org-proxy to a more generic project_browser URL

Problem/Motivation

The front end still communicates to the backend via the "drupal-org-proxy" endpoints. This was set in the very early days, but now that we have multiple sources, with `drupalorg` being just one of them, that URL doesn't seem adequate for "core".

Proposed resolution

I suggest changing it to "project_browser_json" or "project_browser_data" or something similar.