Add context to community ads

[cellio]

https://meta.codidact.com/posts/280055

Community ads that promote questions have good presentation -- the ad asks "could you answer this question?" and then has the title of the question. However, some communities have categories that hold things that aren't exactly questions -- challenges (Q&A post types but not questions in the usual sense), resources, recipes, code reviews, golfing challenges, and so on. Can we add some context for these cases?

A minimal approach would be to add the category name to the generated ad somewhere. A better approach would be to allow the text at the top (in the blue section) to be customized in the category settings. It could go next to the existing category setting for whether the category participates in (random) ads.

While one might say "well, don't nominate those posts for cross-community ads, then", keep in mind that we also support random ads for use outside our network, and the situation could arise there too.