Show all matching labels when using /label quick action

Update from @donaldcook: For first iteration, let's bump limit up from 5 to 20.

Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/70387

Description

Slash commands menu for labels is showing only 5 labels. We have more than 5 labels with naming "Waiting - xxx". I can choose only between the first 5 with slash command, see the screenshots. If I type /label ~WAITING and press spacebar, the context menu disappears. If I type /label ~"WAITING the context menu doesn't open at all.

Proposal

Show all labels matching the slash command value.

Screenshots