Replace Search::Index with Search::Records

Lean on ActiveRecord models for searching and strip out the raw SQL.
Replaces the search_index_* tables with sharded search_records_* tables
as that allows us to use a Search::Record model name.

A Class is dynamically created for each record table shard so that we
and we can access it via the Search::Record.for_account(account_id)
method.

lib/tasks/search.rake

@@ -1,9 +1,9 @@
 namespace :search do
   desc "Reindex all cards and comments in the search index"
   task reindex: :environment do
-    puts "Clearing search index shards..."
-    16.times do |i|
-      ActiveRecord::Base.connection.execute("DELETE FROM search_index_#{i}")
+    puts "Clearing search records..."
+    Search::Record::SHARD_COUNT.times do |shard_id|
+      ActiveRecord::Base.connection.execute("DELETE FROM search_records_#{shard_id}")
     end
 
     puts "Reindexing cards..."