Added form-enabling modules to Post model per Section 6.1.

This is a bit of a no-op at the moment, but got it out of the way as it
is the last standalone task on the list for Sections 6 and 6.1.

RSpec: 58 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 20 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 1