Refactor get tagline script so it doesn't require modules to have a dashboard type of other

We want to be able to have a configured tagline without forcing a specific dashboard type, so we add a fallback of just having a tagline property on the model.

Ideally we'd also get rid of the "other" stuff, but it stays for backwards compatibility.
Add support for high-volume-transaction option as dashboard-type
Merge pull request #414 from alphagov/refactor/get-tagline

Refactor/get tagline