v2.2.0: Add support for label merge

There are two scenarios where handling a label configuration would require merging one label into another:

- The configured label exists in the current labels and its alias exists in the current labels
- Multiple aliases of a configured label exist in the current labels

Previously, "GitHub Label Sync" did not have support for label merge, and would return an error when the user's
configuration would have required a merge operation.

The merge capability is added here.

The GitHub labels API does not provide a merge capability, so merging labels requires the use of the issues API to
add the "expected" label to each issue/PR that has the "actual" label.

From the user's perspective, the outcome of this "merge" operation is no different from any simple "changed" operation:
the "actual" label has been replaced by the "expected" label in the repository labels and all issues/PRs the label was
applied to.