This is deliberately a serial queue.

This commit creates a new operation that validates a batch. It moves
all of the validation error handling out of ::CreateBatch, leaving that
operation quite simple.

This commmit takes the validation and concurrency control out of create_students_job
and puts it in school_students_controller.

The idea is that instead of validating then committing one job of 50
students, we:

1. validate the entire batch of N students quickly by calling
   SchoolStudents::ValidateBatch
2. Split the batch into chunks of 50.
3. Enqueue them in the context of a GoodJob::Batch, ensuring atomic
   enqueue of all `N/50` jobs.
4. Control concurrency by not allowing creation of another Batch whose
   description field matches the school ID (this makes up for the fact
   that GoodJob::Batch does not have a concurrency key like Jobs do).

Switching to `filter_map` ensures that a parameter list of `[]` gets
through as `[]` and not `[nil]`.

The validation functionality has been refactored and raised to the
controller level that calls CreateBatch, so the tests are at a higher
level now.

Recent updates in profile made a small number of changes to the
structure of errors and parameters in profile. This commit brings
editor-api up to match it.

This commit moves the method SchoolStudentsController#batch_in_progress?
to School#import_in_progress? and changes the batch identifier from
"school_id:#{school.id}" to simply "#{school.id}".

This allows us to expose the import_in_progress field through the API so
that we can enable/disable front-end UI components to prevent user
frustration.

This change includes the state of current imports for a school in the
API response.

This commit changes UserJob from having a 1:1 relationship with
GoodJob::Job to tracking the ID of a GoodJob::Batch.

This means that the UserJobsController had to change to track the state
of a batch and emulate the status messages that a Job used to provide
(e.g. runnning, queued, etc.).

The front end can now poll for the status of this job.

We were only decrypting students at validation and not creation. Fix.