Add configurable build message

Having a hard-coded HTML description is pretty inconvenient when you don't want to allow raw HTML descriptions, or if you'd rather have an altogether different build description message. This pull request adds a customisable template for the build message.
Right now, if you don't touch the config, the form will pull the default from line 291, so everything behaves the exact same as without these commits unless you specifically change the description template.

If you specifically blank the field in the config, you'll have an empty description — which is the behaviour I'd expect it to have, and consistent with all the other fields as well.