Show only applicable labels for the `/label` quick action

Description

The /label and /unlabel slash commands should show only applicable labels in their autocomplete list. This would allow for even quicker editing of labels, shaving some time from searching the autocomplete list.

Proposal

As an example, if the issue/MR has only one label, such as ~"feature proposal":

• when typing /label don't show the ~"feature proposal" label in the autocomplete list
• when typing /unlabel show only the ~"feature proposal" label in the autocomplete list