Report scheduled time of next changeset sync

Created by: ryanslade

We should allow clients to find out when the next scheduled sync is for a specific changeset.

We can expose it in the GraphQL API as follows:

type ChangesetSyncSchedule {
    # Time of next scheduled sync
    scheduled: DateTime
    # Position in queue, 0 indexed
    position: Int
}

type Query {
    # Changeset Sync Schedule data
    changesetSyncSchedule(changesetID: ID!): ChangesetSyncSchedule
}