Use Rails transactional tests

As of Rails 5.1:

  - Rails has built-in support for running tests within database
    transactions, so Database Cleaner is no longer needed for this.
  - Rails automatically shares the database connection across threads,
    so transactional_capybara is no longer needed.

Changes:

  - Enable `use_transactional_tests`.
  - Remove transactional_capybara.
  - Remove the Database Cleaner test wrapper.

This resolves:

  - transactional_capybara mismanages the shared database connection,
    causing the connection to falsely report as idle after the first
    test. At 6 minutes (idle_timeout + reaping_frequency) into testing,
    the connection is closed, causing e.g. `PG::ConnectionBad` errors.

spec/spec_helper.rb

@@ -20,9 +20,6 @@ require 'webdrivers'
 # all migrations applied
 ActiveRecord::Migration.maintain_test_schema!
 
-# Keep capybara and the database on the same page
-require 'transactional_capybara/rspec'
-
 # Adds rspec helper provided by paper_trail
 # makes it easier to control when PaperTrail is enabled during testing.
 require 'paper_trail/frameworks/rspec'
@@ -49,10 +46,8 @@ RSpec.configure do |config|
   # config.mock_with :flexmock
   # config.mock_with :rr
 
-  # If you're not using ActiveRecord, or you'd prefer not to run each of your
-  # examples within a transaction, remove the following line or assign false
-  # instead of true.
-  config.use_transactional_fixtures = false
+  # Test within database transactions
+  config.use_transactional_examples = true
 
   # Run specs in random order to surface order dependencies. If you find an
   # order dependency and want to debug it, you can fix the order by providing