Categories can be abused

[SweetJonnySauce]

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

[inutt]

Oops, that's my plugin - looks like I accidentally set the category field to a string instead of an array. I'll fix that (and the overly specific last updated field) and submit an update.

[inutt]

If it helps with any automation, here's a schema I've written for validating plugin json files. It doesn't currently validate licence strings as I couldn't find a reasonable way to check the list of those, but I seems to work fairly well for all the combinations of invalid data I've tried so far.

plugin.schema.json

[SweetJonnySauce]

Thanks for the schema. this'll be helpful.

merge.yml already calls api.github.com and the License is available there so that would be a simple check/validation.