Add available on edx.org course search filter

[igobranco]

On course search page add a search filter that allows to view all courses that are available only on edx.org.

Use platform categories to show a course filter. Like edulib: https://catalogue.edulib.org/en/cours/?limit=21&offset=0
https://www.nau.edu.pt/pt/categorias/plataformas/

Alternatively on upstream add a course run elastic search document a new field "LMS Backend" when indexing. And add LMS Backend has a new search filter.