[BUG] extraneous information in ad markup

[jeremybmerrill]

Describe the bug
Related to #17, the markup of the ad that's sent to the server contains extraneous information.

To Reproduce
Steps to reproduce the behavior:

1. Follow steps from 17.
2. Examine HTML in items[0].payload.adTargetingHtml; ctrl-F for likes.

Expected behavior
We should include only the ad itself.

Additional context

I think the problem is solvable by treating the innermost .userContentWrapper as the ad markup. (Or, at least, only sending that.) That's what we've done in the old extension. I suspect this excludes stuff like FB's "Coronavirus Information" banner too. Haven't checked and I'm not sure if that's good or not.