Feature spec proving a "member" can create a valid image post.

Required some additional `require`s for our recently-added PostDecorator
class and support classes, since RSpec autoloading differs from Rails'.
Pfffft.

RSpec: 128 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 44 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5