Replace category system with tagging system

We're currently limited to categorizing crates by their most prominent feature, but not all crates fit well under our current set of categories (looking at you, "Helpers" category), and some might fit more accurately under multiple categories.

Therefore, we should replace our current category system with a tagging system, so that crates can be tagged by ALL of their features (or by their intersection of features).

No files, tests, or entry points are identified in the issue. Start by locating the current category definitions and every place categories are stored, edited, filtered, and displayed; then establish the tagging model and migration scope. Done means crates can carry multiple tags and the existing category-based workflow is replaced consistently.

Written by the indexing model from the issue text.