LIMS-2136: Fix editing priority processing pipeline

[ndg63276]

JIRA ticket: LIMS-2136

Summary:

At the moment you cannot edit the priority processing pipeline.

Changes:

• Display the priority pipeline even if not yet selected
• Use the pipeline id as the model for the dropdown, not the pipeline name

To test:

• Go to a puck with no priority pipeline set, eg /containers/cid/359802
• Check there is a field to set the priority pipeline, and it says "Click to Edit" initially
• Choose a pipeline, check a notification appears to say it has been successfully updated
• Refresh the page, check the chosen pipeline appears
• Edit again and check it remains after another refresh