Add back partial support for twitter.com using the old twitter mobile UI

On June 1st 2020 twitter.com completely disabled serving the legacy UI
which tweeper kept supporting using a User-Agent trick.

The new official UI uses retrieves json after authenticating with
cookies and generates the HTML client-side, so it's too complicated for
the current Tweeper structure.

Work around the issue with the help of another User-Agent trick, pretend
to be an old Android phone, which makes tweeper serve the old mobile UI
which can be easily scraped by tweeper.

This approach looses support for some functionalities like embedded
media but at least makes Tweeper work again with twitter.com