FIX: Add DB constraints for post & topic counter cache for UserStat (#15626)

Ensures that `UserStat#post_count` and `UserStat#topic_count` does not
go below 0. When it does like it did now, we tend to have bugs in our
code since we're usually coding with the assumption that the count isn't
negative.

In order to support the constraints, our post and topic fabricators in
tests will now automatically increment the count for the respective
user's `UserStat` as well. We have to do this because our fabricators
bypasss `PostCreator` which holds the responsibility of updating `UserStat#post_count` and
`UserStat#topic_count`.

spec/components/guardian_spec.rb

@@ -1184,6 +1184,8 @@ describe Guardian do
   end
 
   describe "can_recover_topic?" do
+    fab!(:topic) { Fabricate(:topic, user: user) }
+    fab!(:post) { Fabricate(:post, user: user, topic: topic) }
 
     it "returns false for a nil user" do
       expect(Guardian.new(nil).can_recover_topic?(topic)).to be_falsey
@@ -1198,11 +1200,6 @@ describe Guardian do
     end
 
     context 'as a moderator' do
-      before do
-        topic.save!
-        post.save!
-      end
-
       describe 'when post has been deleted' do
         it "should return the right value" do
           expect(Guardian.new(moderator).can_recover_topic?(topic)).to be_falsey
@@ -1227,9 +1224,6 @@ describe Guardian do
       fab!(:group_user) { Fabricate(:group_user) }
 
       before do
-        topic.save!
-        post.save!
-
         SiteSetting.enable_category_group_moderation = true
         PostDestroyer.new(moderator, topic.first_post).destroy
         topic.reload
@@ -1262,10 +1256,8 @@ describe Guardian do
     end
 
     context 'as a moderator' do
-      before do
-        topic.save!
-        post.save!
-      end
+      fab!(:topic) { Fabricate(:topic, user: user) }
+      fab!(:post) { Fabricate(:post, user: user, topic: topic) }
 
       describe 'when post has been deleted' do
         it "should return the right value" do