Delay search indexing to work around transaction

Scheduled publishing is currently performed every 15 minutes across all
backend machines (keeps all machines equal and introduces redundancy
into the process) using a rake task. Each due edition is published by
EditionPublishingWorker, which performs the action with a transaction
lock so that only one backend machine will actually succeed if more than
one attempts to publish the same edition. The issue is that whilst the
transation is in progress, the edition will remain in a "scheduled"
state in the database. And because search indexing is performed
asynchronously by sidekiq, it is sometimes the case that the search
indexing job will be picked up and attempted before the transaction can
complete. This results in the edition not being indexed, as
SearchIndexAddWorker will not index a non-"published" edition.

This change introduces a 10 second delay to the search indexing job to
give the transaction enough time to complete.

Note that this is a temporary measure to get scheduled editions indexing
reliably. A story is to be spun up for a fuller solution that doesn't
rely on a transaction lock when publishing scheduled editions.
Move indexing listener to back of the queue

To minimise the chance that the search indexing job is picked up before
a scheduled-edition publishing transaction is complete, we move have the
search indexer listener register itself last.
Merge pull request #1475 from alphagov/delay-search-indexing

Delay search indexing to work around transaction