Add a "licence" filter on the search page

[sampaccoud]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe. On the search page, we would like to able to filter courses by the licence of their content.

Describe the solution you'd like

• [ ] Add a "short_name" field to the Licence model for use on the filter. (we could set it to values of the form CC-BY, CC-BY-NC, etc.)
• [x] Add license field on the course search index using "short_name" as a token
• [x] Add a licence filter to the search page

[mbenadda]

Done in #1728

[mbenadda]

As it turns out, we might really need to add a short name just for use as a human name instead of the actual title. As for the token, we can keep using the ID.