Celery task structure improvements

[brianhelba]

• Add missing __init__.py files to tasks
  • These are essential to making these into actual importable modules. Otherwise, they will be considered to be "implicit namespace modules", which have different behavior in subtle ways.
• Move tasks below the bats_ai.core package
  • This is the canonical location for Celery tasks in Resonant.
  • This is necessary to make app.autodiscover_tasks() work properly, as it scans the INSTALLED_APPS (in this case, core) for a tasks module. Perhaps this also wasn't working before due to the lack of __init__.py, so tasks wasn't considered a real module.
• Use @app.task instead of @shared_task
  • Previously, @shared_task was preferred by Resonant, but I realized that @app.task will be able to provide better typechecking. @shared_task is essential only for reusable apps (and core isn't reusable). I'll make an upstream change to the Resonant cookiecutter to prefer @app.task too.

[BryonLewis]

LGTM - did a quick onceover, pulled and tested some tasks and everything seems to work well.
We'll be switching a lot of internal tasks to using a new tool created by vision (BatBot) for spectrogram generation anyways, I'll clean up then some of the mess of ./tasks/tasks and other stuff. Should be simplified a bit using BatBot.

[brianhelba]

@naglepuff I'd suggest just looking at the description of this PR, to be aware of the rationale.