Consolidate standard library requirements into an initializer.
Extract characteristics common to NYPL pages into a mixin.
Rename methods named "mocks" to "stubs".

According to Martin Fowler, the word "mock" implies that the test object
has expectations of its own. In this case, it's simply a stand-in for
what would otherwise be an HTTP request.
Change order of execution, add comments for clarity.
Add NYPL program and exhibition content items.
Add tests for NYPL programs and exhibitions.
Remove redundant indices.

Unbeknownst to me, adding unique constraints in PostgreSQL creates an
index, so there's no need to create another one.
Add "Events" to navigation.
Hide program thumbnails if they're the default one.
Add "Configuration" and "Updating" sections to README.