search: fix underestimation of page count for blank searches.

Bug: In Postgres 13, getting the count of a blank search underestimated
the page count by a large margin (~700,000 posts).

The query we were executing was this:

    EXPLAIN (FORMAT JSON) SELECT * FROM posts ORDER BY id DESC

The `ORDER BY id DESC` clause triggered a parallel seq scan query plan
in Postgres 13, which for some reason causes Postgres to underestimate
the row count by large amount in each parallel branch.

Getting rid of the ORDER BY clause makes it do a regular seq scan, which
gives an accurate estimate.

app/logical/explain_parser.rb

@@ -1,5 +1,10 @@
-class ExplainParser < Struct.new(:sql)
+class ExplainParser
   extend Memoist
+  attr_reader :relation
+
+  def initialize(relation)
+    @relation = relation
+  end
 
   def query_plan
     result = ApplicationRecord.connection.select_one("EXPLAIN (FORMAT JSON) #{sql}")
@@ -11,5 +16,9 @@ class ExplainParser < Struct.new(:sql)
     query_plan["Plan Rows"]
   end
 
+  def sql
+    relation.reorder(nil).to_sql
+  end
+
   memoize :query_plan
 end

app/logical/post_query_builder.rb

@@ -851,7 +851,7 @@ class PostQueryBuilder
     end
 
     def estimated_row_count
-      ExplainParser.new(build.to_sql).row_count
+      ExplainParser.new(build).row_count
     end
 
     def cached_count