Allow user to set default Proof notifications

[#1]

The documented default for Proof is to send a daily summary of comments. For users managing dozens of proofs at a time, this is bogging down their email. I think there are a variety of ways to customize this experience: allow the user to set defaults for when comments come in, decisions are made, or they're mentioned. Related: It would help if the admin could set or even lock defaults for the above options based on access level.