Fix #4178: add ability to mass undo tag edits.

Adds checkboxes to the /post_versions index allowing you to select and
undo multiple versions at once.

test/test_helpers/system_test_helper.rb

@@ -6,4 +6,11 @@ module SystemTestHelper
     fill_in "Password confirmation", with: password
     click_button "Sign up"
   end
+
+  def signin(user)
+    visit new_session_path
+    fill_in "Name", with: user.name
+    fill_in "Password", with: user.password
+    click_button "Submit"
+  end
 end