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/system/post_version_test.rb

@@ -0,0 +1,33 @@
+require "application_system_test_case"
+
+class PostVersionTest < ApplicationSystemTestCase
+  context "Post versions" do
+    setup do
+      @user = create(:builder_user)
+
+      as @user do
+        @post = create(:post, tag_string: "tagme")
+        travel 2.hours
+        @post.update!(tag_string: "touhou")
+        travel 2.hours
+        @post.update!(tag_string: "touhou bkub")
+        travel 2.hours
+      end
+
+      signin @user
+      visit post_versions_path
+    end
+
+    context "clicking the undo selected button" do
+      should "undo all selected post versions" do
+        check id: "post-version-select-all-checkbox"
+        assert all("td .post-version-select-checkbox:not(:disabled)").all?(&:checked?)
+
+        click_link "subnav-undo-selected-link"
+        assert_selector "#notice span.prose", text: "2/2 changes undone."
+
+        assert_equal("tagme", @post.reload.tag_string)
+      end
+    end
+  end
+end

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