refs https://github.com/TryGhost/Team/issues/1163
This allows users to not provide a title for an Offer. We store the lack
of a title as `NULL` in the DB, but we will always provide a string to
the API so that the title can safely be used in HTML.
no-issue
This adds the concept of "Value Objects" to an Offers properties,
allowing us to move validation out and ensure that an Offer will only
ever have valid properties, without having to duplicate checks - or
leave them to the persistent layer. This means we can fail early, as
well as write unit tests for all of our validation.
