Updating document or image triggers a build

### As a resident, I want to be able to access up-to-date documents and see the latest image, so that I access the right information.

### As an author, I want to be able to update/edit a document or image in Joplin and have that immediately publish out onto Janis, so that I can ensure my audience accesses the right thing.

Updating/editing a document or an image in Joplin currently does not trigger a build. But it should.

**Acceptance criteria**
- [ ] When a user edits an image ([such as here](http://joplin-staging.herokuapp.com/admin/images/29/)) by changing the file, that image will update wherever it is live across austin.gov
- [ ] When a user edits a document ([such as here](http://joplin-staging.herokuapp.com/admin/documents/edit/47/)) by changing the file, that document will update wherever it is live across austin.gov