Final edits to batch assign action

[sianteesdale]

Data Template

Ticket

• Link: Decide frequency and timing of running simple batch assign config-manager#143

Additional information:
This PR does the final edits required for the batch assign Github action. The following changes have been made:

• Update scheduling to 7pm UTC (8pm BST), to reflect Github normally triggering actions an hour late
• Add Slack notifications for when the action fails
• If single-source is selected, and all batches have been processed, do not hard fail the action. Instead, exit quietly.
• Moved the Python script to the .github/scripts folder, where other Github action Python scripts exist
• Update tests with the new location of the Python script