- Refactor uploads manager
- Refactor importer, update import spec
- Add more object storage specs

`perform_enqueued_jobs` is a Sidekiq method.
Using this method violates the Dependency inversion principle[0].

This commit replaces `perform_enqueued_jobs` with ActiveJob's abstract
method `perform_enqueued_jobs` in specs.

[0]: https://en.wikipedia.org/wiki/Dependency_inversion_principle

This overrides values defined in the project JSON with the values
provided in project.import_data.data['override_params'].

These could be passed from the API.

This means import data doesn't necessarily have to have an import_url
anymore.

The `ProjectImportData` could just contain the override data in it's
serialized data attribute.

The import data is automatically cleaned up after it is finished by
the state machine.