Automatically pass desired changeset comment to iD editor

NOTE: the iD side of it is done <https://github.com/openstreetmap/iD/issues/2311>
and is not yet live on OSM website, but will be when next iD version
comes out.
Make sure python also escapes any slashes in changeset comment prefill
Merge pull request #318 from danstowell/topic/commenttoid

Automatically pass desired changeset comment to iD editor