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.
If you have the plugin configured and upgrade with this change set, the default build description is the same as the old hardcoded one, so everything will be unchanged (that's how it behaved when I deployed it locally). The field in the config will be automatically populated with the correct value.

See, for reference, how `whitelistPhrase` is handled.