Gather metrics on what versions of PDF.js are being used with Hypothesis

Related to https://github.com/hypothesis/client/issues/2643, it would be useful for the Hypothesis client to report to us which versions of Hypothesis it is being used with, so that we can make decisions about spending resources to support different versions.

Some ideas about how we might do this:

1. Add informational query parameters to an existing request that the client makes
2. Add a new `/api/metrics` or `/api/events` API call to h which the client can use to report various metrics or usage events

We used to use Google Analytics but stopped doing that because we were hitting usage limits and the data that came out of it was not super valuable. Also Google Analytics is increasingly being blocked by various browsers to prevent cross-site tracking.