Cannot create a label from the group board sidebar

Label management should work like this in the group board (and in particular, the sidebar: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2467#note_38452577).

As shown in the gif, you cannot create a label from the sidebar currently.

added regression label

added frontend label

@psimyn - @felipe_artur is out on Friday (it's his birthday!) so could you take the first look please?

assigned to @psimyn

mentioned in merge request !2869

mentioned in commit 2c2b6b9a

assigned to @sstern and unassigned @psimyn

assigned to @psimyn and unassigned @sstern

unassigned @psimyn

assigned to @sstern

unassigned @sstern

assigned to @sstern

added 2fa label and removed frontend label

added AP1 AP2 AP3 labels and removed 2fa label

added testblack label

added testred label and removed testblack label

removed issue boards label