Add `campaigns add-changesets` subcommand

Created by: mrnugget

This adds the changesets with the given external IDs to the given campaign.

Example:

$ src campaigns add-changesets -campaign-id Q2FtcGFpZ246NA== -repo-name github.com/sourcegraph/sourcegraph 5016 5017 5018
Created 3 changesets.
Added changeset to campaign. Changesets now in campaign: 10

Current downsides

(These are mostly due to how our API works with batches and could be fixed on the API side, collecting these just here for documentation/transparency)

• When one of the changeset already exists, the command fails with:

[
  {
    "message": "pq: duplicate key value violates unique constraint \"changesets_repo_external_id_unique\"",
    "path": [
      "createChangesets"
    ]
  }
]

• When one of the external IDs specified is invalid (non existant, for example), it creates the rest silently but returns an error message for the single invalid one:

[
  {
    "message": "error in GraphQL response: Could not resolve to a PullRequest with the number of 5013.",
    "path": [
      "createChangesets"
    ]
  }
]