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/features/commercials_spec.rb

@@ -85,7 +85,6 @@ feature Commercial do
       click_button 'Update'
       page.find('#flash')
       expect(flash).to eq('Commercial was successfully updated.')
-      TransactionalCapybara::AjaxHelpers.wait_for_ajax(page)
       expect(event.commercials.count).to eq(1)
       commercial.reload
       expect(commercial.url).to eq('https://www.youtube.com/watch?v=M9bq_alk-sw')
@@ -118,7 +117,6 @@ feature Commercial do
       end
       page.find('#flash')
       expect(flash).to eq('Commercial was successfully destroyed.')
-      TransactionalCapybara::AjaxHelpers.wait_for_ajax(page)
       expect(event.commercials.count).to eq(0)
     end
   end

spec/features/proposals_spec.rb

@@ -138,7 +138,6 @@ feature Event do
 
       page.find('#flash')
       expect(page).to have_content 'Proposal was successfully submitted.'
-      TransactionalCapybara::AjaxHelpers.wait_for_ajax(page)
       expect(current_path).to eq(conference_program_proposals_path(conference.short_title))
       expect(Event.count).to eq(expected_count)
     end

spec/features/versions_spec.rb

@@ -392,7 +392,7 @@ feature 'Version' do
     click_link 'Comments (0)'
     fill_in 'comment_body', with: 'Sample comment'
     click_button 'Add Comment'
-    TransactionalCapybara::AjaxHelpers.wait_for_ajax(page)
+    expect(page).to have_text('Comments (1)')
     Comment.last.destroy
     PaperTrail::Version.last.reify.save