This PR adds a composite index (:account_id, :content_type, :created_at)
on the table messages.
This index is added as a temporary fix for performance issues in the
CSAT responses controller where we query messages with account_id,
content_type and created_at. The current implementation
(account.message.input_csat.count) times out with millions of messages.
TODO: Create a dedicated csat_survey table and add entries when surveys
are sent, then query this table instead of the entire messages table for
better performance.
eea1ab3002