Bug: Alert updates not send when changing alert collection of a fridge

Expected Behavior

After changing an alert collection of a fridge, I expect an alert update to be initiated when CryoRelay polls for new updates.

Current Behavior

The expected behaviour does not occur

Possible Solution

Update the get method of AlertDefinitionView to also consider changes to Fridge.alert_def

Steps to Reproduce

• Change a Fridge.alert_def from one AlertCollection to another
• Observe that, when a CryoRelay polls for alert updates, no updates will be returned

Context (Environment) and Detailed Description

When a CryoRelay performs a poll to determine if any of its fridges need an alert update, CryoAdmin should return the list of current alert rules for each fridge, if this list has changed since the last time the CryoRelay polled. However, if the Fridge.alert_def has been changed from one AlertCollection to another, CryoAdmin does not detect this as a modification and hence won't return the updated list when the CryoRelay polls for it.

Possible Implementation

Keep track of datetime when Fridge.alert_def was last modified in addition to when the entire model was last modified