Change how service requests A+ to show notifications

[murhum1]

Description

What?

Changed the behavior of sending notifications to students when editing feedback messages. Now emptying an existing feedback message removes existing notifications, changing an empty feedback message to a non-empty one creates a notification, and editing a non-empty feedback message to another non-empty ones preserves the notification status in A+ e.g. if a student has read the notification already, they won't get a new notification of the edited message, and if they haven't, the notification is not removed.

Why?

Requested by teachers.

Fixes #126 and #127, requires apluslms/a-plus#1500 on the A+ side.

Testing

Remember to add or update unit tests for new features and changes.

• How to test your changes in A-plus
• How to test accessibility

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Selenium tests.
• Other test. (Add a description below)
• Manual testing.

Tried out varying cases of editing feedback messages.