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
Use send_task to dispatch tasks from backdrop.write

Instead of importing the tasks directly, we can use send_task
and a string as the task name. Only the worker needs to import
the tasks directly, so they have been moved back to the
transformers module.
Merge pull request #382 from alphagov/emit-write-messages

Use send_task to dispatch tasks from backdrop.write