Merge pull request #379 from alphagov/emit-write-messages

Use write API config for celery tasks
Use transformer config for tasks

Although there's an implicit dependency on backdrop.write's config (they
must be configured to use the same AMQP URL), it's too fragile to
depend on config deployed by a different fabric task.

We specify config both in tasks and the worker so that they can be more
easily split in future.
Merge pull request #380 from alphagov/emit-write-messages

Use transformer config for tasks
Move tasks to backdrop.write

Tasks depend on backdrop.write's config. This is a temporary fix for
config issues caused by having backdrop.write and backdrop.transformers
depend on separate config. This will be refactored when the worker
codebase is extracted from backdrop.
Merge pull request #381 from alphagov/emit-write-messages

Move tasks to backdrop.write