add phone endpoint for form submissions by user

Nothing's hitting this yet—next step is going to be to add an invisible option to the app builder, but putting this up will let @ctsims test it. URL is /a/<domain>/phone/historical_forms/ with digest auth (or a logged-in user).
That's a good question. I think we can leave it the way I have it here and the reasoning goes like this. `self._xml_string_to_element` crashes if you pass it `None`, so this function remains unchanged except that in some situations when it used to crash with a meaningless error it now returns `None`. Thus in all cases where it now crashes, it also used to crash—the crash may have just been pushed out to a different place.

The reason I needed this was that in historical forms, it iterates over all forms' xml (are rare thing for us to do); it's possible that some malformed forms in the past will be missing those (there were some on my computer but that's likely from me having messed around in the DB).