How to properly alter the database?

[icculus (Ryan C. Gordon)]

@icculus

(Specifically, I was thinking of adding a means to specify an “email subject slug” per category, so emails can be sent with that instead of a category’s full title, but I’m sure I’ll eventually have 12 other unrelated ideas that want to change the schema too, heh)

…and now I have done that, in case anyone wants to laugh at my PR.

Add an optional category-specific [slug] for email subject lines.