a8n/core: Expose labels of all changesets in campaign in GraphQL API

[ryanslade]

From RFC 20

We want to sync labels on changesets in one direction so that they appear on the campaign page.

[eseliger]

Do you think this will suffice?

type ChangesetLabel {
	text: String!
	color: String # leaving this optional for code-hosts that don't have coloring (TODO: do such exist?)
	description: String # github has optional descriptions for labels, do we want to support that either?
}
type ChangesetLabelConnection {
	totalCount: Int!
	nodes: [Label!]!
}
type ExternalChangeset {
//	...
	labels: LabelConnection!
//	...
}