Add support to only return published forms from the backend

Forms can be marked as published (i.e., should be in active use) or unpublished (i.e., draft forms that are available in the system but should not really be displayed to users). Currently, there is no way to request only the published or unpublished forms via the REST API.

It would be nice to expose:

• FormService#getPublishedForms()
• Some variation of ForService#getForms() to allow searching by form and publication status.