Use transactions to apply web UI SSH key edits

Summary:
Ref T10917. Converts web UI edits to transactions.

This is about 95% "the right way", and then I cheated on the last 5% instead of building a real EditEngine. We don't need it for anything else right now and some of the dialog workflows here are a little weird so I'm just planning to skip it for the moment unless it ends up being easier to do after the next phase (mail notifications) or something like that.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10917

Differential Revision: https://secure.phabricator.com/D15947