RapidWords API endpoint for semantic explorer

[fbanados]

As we add the rapidwords categorization to our main database, we want to be able to use a faster API endpoint to deploy to both the current and new mobile-aware versions of semantic-explorer. This issue is intended to guide discussion and design with students.

The key idea now would be to have an endpoint such that, for example, https://itwewina.altlab.dev/api/rapidwords-index/?rw_index=1.1 provides a JSON with a list of entries that can be visualized in the interface.

Although eventually we want to minimize the size of the JSON response as well (thus not including data that is not used), a first approach is to make use of the new indexing for RapidWords entires and see if that makes things fast enough for now.