campaigns: add backlink when creating changesets

Created by: LawnGnome

Here's how the backlinks look in GitLab, as an example:

The styling is taken from how GitHub styles the line added when an issue is created from a comment.

There's also a bonus refactor in here: #13965 added internal/db/namespaces.go: I've expanded this out to add the functionality I need (resolving a namespace from an ID tuple) and also brought that file more into line with other files in internal/db by using sqlf and namespacing its methods with an empty struct like the other files. Those are in separate commits, so if people would prefer these to go into separate PRs, I'm happy to make that split.