Send ajax request for label update only if they are changed

## What does this MR do?

At the moment we send requests to update the labels on issue even if nothing has been changed. This fixes that and only sends requests when something has actually been changed.

The code gets the initial selected values based on the inputs & then compares that to the selected values when submitting. If they are equal, then no request is sent.

## What are the relevant issue numbers?

Closes #19472 

See merge request !5071