FEATURE: allow a huge number of users to share common prefix

Previously username suggester would give up after 100 attempts at getting
a username and fallback to random string.

This amends the logic so we do all the work of figuring out a good username
in SQL and avoids a large amount of queries in cases where a lot of usernames
were used up.

This corrects an issue on sites with large numbers of anon users

spec/components/user_name_suggester_spec.rb

@@ -10,6 +10,15 @@ describe UserNameSuggester do
       SiteSetting.max_username_length = 15
     end
 
+    it "keeps adding numbers to the username" do
+      Fabricate(:user, username: 'sam')
+      Fabricate(:user, username: 'sAm1')
+      Fabricate(:user, username: 'sam2')
+      Fabricate(:user, username: 'sam4')
+
+      expect(UserNameSuggester.suggest('saM')).to eq('saM3')
+    end
+
     it "doesn't raise an error on nil username" do
       expect(UserNameSuggester.suggest(nil)).to eq(nil)
     end