Primarily this is in tests (which were caught by temporarily
introducing acts_as_tenant and enabling safety checks), but notably
action cable connections were not working properly, and that's now
fixed.
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.