Shard the search index into 16 tables

Create search_index_0 to search_index_15 tables and shard each index by
account id. MySQL has no ability to pre-filter fulltext indexes by
another field so this is the best bet for improving performance.

Each fulltest index internally creates 11 sub tables (see
https://dev.mysql.com/doc/refman/8.4/en/innodb-fulltext-index.html) so
actually we have 192 tables in total here.

The search_index table name is generated dynamically based on the
account_id.

lib/tasks/search.rake

@@ -1,8 +1,10 @@
 namespace :search do
   desc "Reindex all cards and comments in the search index"
   task reindex: :environment do
-    puts "Clearing search index..."
-    ActiveRecord::Base.connection.execute("DELETE FROM search_index")
+    puts "Clearing search index shards..."
+    16.times do |i|
+      ActiveRecord::Base.connection.execute("DELETE FROM search_index_#{i}")
+    end
 
     puts "Reindexing cards..."
     Card.find_each do |card|