Categories can be abused

Malformed categories can be submitted and populate the dropdown box with cruft.

I'm happy to take a stab at a PR for this. Thinking:

1) a CI check
2) static list for dropdown box

Start by inspecting how submitted categories are stored, validated, and exposed to the plugin browser, then review the repository's CI configuration. Define the accepted category source and the validation check, and confirm that malformed categories no longer populate the dropdown.

Written by the indexing model from the issue text.