FIX: Select earliest post when aggregating posts in a topic for search.

This is a revert of
d8c796bc44
and
5bf0a0893b.

Linking to the post within a topic that has the highest rank was
confusing users and hard to explain because ranking is determined via
the PG ranking function. See the following meta topics for the
complaints after we switch to the new ordering:

1. https://meta.discourse.org/t/title-search-not-working-as-expected/157737
2. https://meta.discourse.org/t/search-results-should-prioritize-first-post-in-topic-when-title-matches-search-term/175154

spec/components/search_spec.rb

@@ -601,8 +601,8 @@ describe Search do
       expect(result.posts.pluck(:id)).to eq([post2.id, post.id])
     end
 
-    it 'aggregates searches in a topic by returning the post with the highest rank' do
-      _post = Fabricate(:post, topic: topic, raw: "this is a play post")
+    it 'aggregates searches in a topic by returning the post with the lowest post number' do
+      post = Fabricate(:post, topic: topic, raw: "this is a play post")
       post2 = Fabricate(:post, topic: topic, raw: "play play playing played play")
       post3 = Fabricate(:post, raw: "this is a play post")
 
@@ -613,7 +613,7 @@ describe Search do
       results = Search.execute('play')
 
       expect(results.posts.map(&:id)).to eq([
-        post2.id,
+        post.id,
         post3.id
       ])
     end
@@ -1892,9 +1892,11 @@ describe Search do
 
     it 'allows to define custom order' do
       expect(Search.new("advanced").execute.posts).to eq([post1, post0])
+
       Search.advanced_order(:chars) do |posts|
-        posts.reorder("(SELECT LENGTH(raw) FROM posts WHERE posts.topic_id = subquery.topic_id) DESC")
+        posts.reorder("MAX(LENGTH(posts.raw)) DESC")
       end
+
       expect(Search.new("advanced order:chars").execute.posts).to eq([post0, post1])
     end
   end