phenocycler_deepcell notification step

[gesinaphillips]

Sends Slack message to #stellar channel if t_maybe_keep_cwl_stellar_pre_convert task succeeds. Tested on DEV.

Reviewing asks:

• Please check that the new notify_user... task is in the right place!
• Unclear on whether Slack or email is more desirable, as adding a Slack notification turned out to be trivial. Happy to switch to or add email notification.

Major changes:

• Replaces prepare_cell_count_cmd EmptyOperator with notify_user_stellar_pre_convert task.
• Made post_to_slack_notify available in utils.py.

Incidental changes:

• Switched to FailureCallback; adapted base callback class to handle dataset_uuid_callable.
• Fixes null case in build_cell_count_cmd (regex str not found in file).
• Standardized imports from utils.py.
• Handling missing values / assuming necessary values (prompting Exception if not present) in trigger_phenocycler.

[sunset666]

I think we would want to keep the prepare steps before every pipeline, just in case we come up with something useful to do there (as we have for the main pipeline). So, instead of replacing it we might want to insert the new task.