Show applied labels at top of label dropdown

Came from this discussion: https://gitlab.com/gitlab-org/gitlab-ce/issues/24774#note_18913985

Proposal

Show selected labels at the top of the label dropdown list. When deselecting a label, it remains in place at the top of the label dropdown list, until the user either:

• closes the dropdown
• or filters the dropdown list

Current	Proposal
Selected labels are just anywhere in the list	Selected labels move to the top of the list