Why this change?
Prior to this change, the ordering of the tags shown in the email subject
was non-deterministic as there was no specific order specified. This
problem was exposed by a flaky test which we had.
What is the fix?
This commit orders the tags used in the email subject first by the
`Tag#public_topic_count` column in descending order and then the `Tag#name`
column in ascending order.
9c9058d0c3