Add account-wide alarms for SES sender reputation

[jameshochadel]

Changes proposed in this pull request:

• Alarms trigger at warning and critical levels if SES reputation metrics are in danger of putting our account under review
• The platform operations team is alerted through our usual notification channels
• Related to Monitor AWS sender reputation product#3214

security considerations

Follows standard approach for CloudWatch alarms.

[bengerman13]

question for all of these, I think:
is the BounceRate cumulative or bucketed? When AWS evaluates whether we're good senders, what window are they looking at?

(reading AWS's docs it sounds like the answer to the second question is "maybe")

[jameshochadel]

The best answer I can find is from this page, which says:

We calculate your bounce rate based on a representative volume of email. A representative volume is an amount of email that represents your typical sending practices. To be fair to both high- and low-volume senders, the representative volume is different for each account and changes as the account's sending patterns change.

So it sounds like they sample delivery notifications. That doesn't explain if the thresholds are instantaneous or averaged over some period of time, though. Are there changes you'd like to see here if it's one or the other?