A tool to do a feature usage survey

Before making breaking changes to Numbas, I'd like to be able to see how many people are using a certain feature. Recent examples include: number-like types, custom functions, custom themes, extensions which add new data types.

I'd like an interface, ideally web-based, which looks at everything in the editor database and lists everything using a selected feature.

Start by reviewing how the editor database stores packages and feature-related data, then identify the entry points for presenting an analysis interface. Done means a web-based tool can select a feature, inspect the database, and list every item using it, including number-like types, custom functions, themes, and extensions that add data types.

Written by the indexing model from the issue text.