Fix unwanted label unassignment

What does this MR do?

• When updating the milestone

  • Do not remove labels when assigning a milestone
  • Do not remove labels when unassigning a milestone
  • Do not remove labels when assigning a milestone and adding another label

• When toggling selected issues labels should be kept

  • Select an issue with an assigned label -> pick another label from dropdown-> unselect the issue -> select the issue again -> submit the form: Existing label should not be removed.

Are there points in the code the reviewer needs to double check?

Labels should not be added or removed to issues when doing bulk actions unless we explicitly select a label from the dropdown

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together