posts: add disapproved:<reason> edit metatag.

* Allow tagging a post with a `disapproved:<disinterest|breaks_rules|poor_quality>` to disapprove it.
* Disallow disapproving active posts.

Fixes #4384.

test/functional/post_disapprovals_controller_test.rb

@@ -29,6 +29,14 @@ class PostDisapprovalsControllerTest < ActionDispatch::IntegrationTest
           assert_response 403
         end
       end
+
+      should "not allow disapproving active posts" do
+        assert_difference("PostDisapproval.count", 0) do
+          @post.update!(is_pending: false)
+          post_auth post_disapprovals_path, @approver, params: { post_disapproval: { post_id: @post.id, reason: "breaks_rules" }, format: "js" }
+          assert_response :success
+        end
+      end
     end
 
     context "index action" do

test/unit/post_test.rb

@@ -1046,6 +1046,38 @@ class PostTest < ActiveSupport::TestCase
           end
         end
 
+        context "for disapproved:<reason>" do
+          should "disapprove the post if the user has permission" do
+            @user = create(:approver)
+
+            as(@user) do
+              @post.update!(is_pending: true)
+              @post.update(tag_string: "aaa disapproved:disinterest")
+            end
+
+            assert_equal(@post.id, PostDisapproval.last.post_id)
+            assert_equal(@user.id, PostDisapproval.last.user_id)
+            assert_equal("disinterest", PostDisapproval.last.reason)
+          end
+
+          should "not disapprove the post if the user is doesn't have permission" do
+            assert_raises(User::PrivilegeError) do
+              @post.update!(is_pending: true)
+              @post.update(tag_string: "aaa disapproved:disinterest")
+            end
+
+            assert_equal(0, @post.disapprovals.count)
+          end
+
+          should "not allow disapproving active posts" do
+            assert_raises(User::PrivilegeError) do
+              @post.update(tag_string: "aaa disapproved:disinterest")
+            end
+
+            assert_equal(0, @post.disapprovals.count)
+          end
+        end
+
         context "for a source" do
           should "set the source with source:foo_bar_baz" do
             @post.update(:tag_string => "source:foo_bar_baz")