Fix #4168: Ignore validations when rejecting tag changes.

* Only check for conflicts with existing aliases/implications when
  requests are created or approved, not when requests are rejected.

* Use `update!(status: "deleted")` instead of `update(status: "deleted")`
  so that if rejecting the request fails we fail immediately instead of
  continuing on and updating the forum topic.

* Wrap `reject!` and `TagChangeRequestPruner.reject_expired` in
  transactions so that if updating either the request or the forum
  fails, they both get rolled back.

test/unit/tag_alias_test.rb

@@ -80,6 +80,16 @@ class TagAliasTest < ActiveSupport::TestCase
       end
     end
 
+    context "#reject!" do
+      should "not be blocked by validations" do
+        ta1 = create(:tag_alias, antecedent_name: "kitty", consequent_name: "kitten", status: "active")
+        ta2 = build(:tag_alias, antecedent_name: "cat", consequent_name: "kitty", status: "pending")
+
+        ta2.reject!
+        assert_equal("deleted", ta2.reload.status)
+      end
+    end
+
     context "on secondary validation" do
       should "warn about missing wiki pages" do
         ti = FactoryBot.build(:tag_alias, antecedent_name: "aaa", consequent_name: "bbb", skip_secondary_validations: false)

test/unit/tag_implication_test.rb

@@ -76,6 +76,16 @@ class TagImplicationTest < ActiveSupport::TestCase
       end
     end
 
+    context "#reject!" do
+      should "not be blocked by alias validations" do
+        ti = create(:tag_implication, antecedent_name: "cat", consequent_name: "animal", status: "pending")
+        ta = create(:tag_alias, antecedent_name: "cat", consequent_name: "kitty", status: "active")
+
+        ti.reject!
+        assert_equal("deleted", ti.reload.status)
+      end
+    end
+
     context "on secondary validation" do
       should "warn if either tag is missing a wiki" do
         ti = FactoryBot.build(:tag_implication, antecedent_name: "aaa", consequent_name: "bbb", skip_secondary_validations: false)