bug 1066058; add Support Reason API crashstorage type

*Preamble: This PR is primarily submitted for discussion purposes (I expect more work to be necessary).*

This adds the `SupportReasonAPIStorage` class which is a sub-class of `BotoS3CrashStorage`; functionally speaking, the former is basically a *highly lossy* implementation of the latter.

cc @twobraids 
I don't understand why we'd ever want buckets separated by time scale. You can set individual S3 items to expire so that you don't need a cleanup task, and if we're going to store per-crash details in S3 I figured that's how it would work.

Although I do wonder if we're storing very small bits of data with relatively constant read/write rates, whether DynamoDB makes more sense for this application. I lack a bunch of context about what this patch actually does ;-)