Migrate to use System Tests

Created by: seanpdoyle

Migrate from the rspec-rails-provided Feature Tests, to the Rails-provided System Tests.

The most beneficial outcome of this migration is the ability to rely on use_transactional_fixtures = true in browser-driven (JavaScript-enabled).

In order to be System Test compatible, this commit replaces test block declarations made with feature with RSpec.describe, along with calls to scenario with it.

In order to continue to support rails@4.2 and rails@5.0, the test harness must elect to run type: :feature tests when [ActionDispatch::SystemTestCase][system-test-care] is not declared.

In those cases, to preserve compatibility:

• pass type: :feature instead of type: :system to tests declared in spec/system
• require database_cleaner gem, along with Feature-test compatible configuration

To achieve this, this commit introduces the SYSTEM_TEST constant, which refers to the correct type: option: :feature for older versions of Rails, :system for newer versions.

Once support for rails@4.2 and rails@5.0 is dropped, these declarations can be removed, and type: SYSTEM_TEST options can be search-and-replaced with type: :system.