Vendor fullscreen on mobile devices

* Why: On mobile devices, having a way to full full screen takes better advantage of the limited real-estate
* Size: M
* Priority: Med (Improvement to usability)
* Partial implementation:
* https://github.com/internetarchive/bookreader/pull/855

We’ve tried to implement this in the past, but the devil is in the details.

First: Add analytics to EnterFullscreen button to see if we have made people more/less likely to use the button.

Possible implementations:
* On small-screened devices, pressing full screen button goes to vendor full screen
* Con: inconsistent; users might be unsure why it went full screen on their device
* Con: Open Library users are sent to the book in theatre mode. They can’t get vendor full screen without minimizing + maximizing, since enter full screen button is an exit full screen button in this mode
* Perhaps if the page starts in theatre mode, have the enter fs button still be displayed