add configuration option for frequency of attestation failure emails

[Sortova]

In the user interface it is possible to set both the attestation time period check (4 hours) and the amount of time without an attestation that can pass before a failure alert is sent (48 hours).

I had my handset off for two days and started receiving attestation failure e-mails every 15 minutes. It would be nice if that was configurable.

[thestinger]

That's why I filed #31. The current implementation of email alerts is basically a proof of concept without throttling.

[thestinger]

The initial throttling is now implemented by 2364354. It isn't currently configurable but rather is hard-wired to an individual device only being able to trigger a single alert every day. They'll still be listed in alerts triggered by other devices within that throttling period. It could be made configurable too, but at least it's not every 15 minutes anymore.