Fix very long listens

#### Normalize listen durations, defang erroneously long listens. …

Sometimes, listens are coming in with extremely long durations.

Basically, in the event that the browser has gone really wonky, we
treat it as a 15 second listen.

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#### Normalize negative listen durations, as well.

There's a (small) chance that some of the extremely high values are so
high because Google Analytics converts from a signed integer to an
unsigned integer, making a negative number into an extremely large positive
number.

This seeks to address that. I added a small window (of under 0.5 seconds)
because it felt right to do so. Not sure if it actually makes sense.