Add API to fetch changesets that are going to be closed by closeCampaign

Created by: eseliger

For the close page, we need to only show the changesets that are owned by the campaign, because only those will be closed. Until we have the delta API, I think a rather simple additional parameter to the changesetsConnection should be sufficient here.

Proposed API design:

diff --git a/cmd/frontend/graphqlbackend/schema.graphql b/cmd/frontend/graphqlbackend/schema.graphql
index 065030bab2..965d4e43d7 100755
--- a/cmd/frontend/graphqlbackend/schema.graphql
+++ b/cmd/frontend/graphqlbackend/schema.graphql
@@ -752,6 +752,8 @@ type Campaign implements Node {
         reviewState: ChangesetReviewState
         # Only include changesets with the given check state.
         checkState: ChangesetCheckState
+        # Only return changesets that have been created by this campaign. Imported changesets will be omitted.
+        onlyCreatedByThisCampaign: Boolean
     ): ChangesetConnection!
 
     # The changeset counts over time, in 1-day intervals backwards from the point in time given in