Implement download-only screenshots with metrics

[ianb]

The plan:

In addition to the "Save" button we have now, we'll have a "Download" button. We might change "Save" to imply "upload" as well (perhaps with an icon).

To start maybe we should just have the image trigger a standard download (to the default location). We'll use the filename scheme we have on downloads from the shot page. (This filename logic should probably be moved from pages/shot/view.js to shared/shot.js)

Of course we'll signal an event in this case, but we should also ensure that we can determine the main experiment: do downloading users engage differently than uploading users? We might want to create metrics specifically to answer that question, unless we figure out how to use a simple event to answer that. We might consider creating a marker in the database to indicate that a download happened so that later we can look at the database to understand long-term user usage.

[ianb]

After b531381 it looks like this now:

It sends an event, addon/download-shot – I'm not sure if this will be enough for us to measure the conversion. We're not storing anything in the database in this case, so /metrics won't work.