edit mutation

Created by: coury-clark

Closes https://github.com/sourcegraph/sourcegraph/issues/35655

This adds support for the edit mutation for the non-global backfill. One big caveat here is that any edit will currently trigger a backfill (including superficial fields such as series label). This isn't ideal, but isn't technically blocking. I've opened https://github.com/sourcegraph/sourcegraph/issues/36499 to address this seperately.

Test plan

Created an insight, let it backfill. Then edited it and watched it generate a new backfill.