Switch to Rspec for testing

I am more familiar with writing tests with Rspec than with the default
Rails implementation. @lencioni is also an Rspec guy so we're switching
over to that instead.

When I was moving over specs, I decided to ditch all the auto-generated
specs. They added little value and for the sake of getting good tests, I
decided it is better we add them manually ourselves.

I stubbed out uploading images to a CDN (Cloudinary) and the whole
Phantomjs process needed to screenshot pages. Testing that part will
have to be manual for a while. Stubbing is dealt with by mocha, also
because of familiarity.
Put `default_scope` in block

These were showing up when running specs as deprecation warnings.
Avoid deprecation warning about locales

This warning was showing up on running rspec:

  [deprecated] I18n.enforce_available_locales will default to true in
  the future. If you really want to skip validation of your locale you
  can set I18n.enforce_available_locales = false to avoid this mess age.
Remove Rails 4 deprecated settings

These settings were generating warnings when running specs.