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).
Is `None` the appropriate "empty" value to use here?  It looks like [this](https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/commtrack/processing.py#L82-L93) is the only other use of this method, and `None` would cause an error there.  I'm not familiar with etree, but it seems like xml elements can be treated as iterables - do you know if there's an etree analog of the empty string that's boolean false?