* [instagram] Add support for stories
Add support for Instagram user's stories
(https://www.instagram.com/stories/<username>/).
First the shared_data in instagram.com/stories/<username> is fetched in
order to retrieve the user_id that is then passed to fetch the stories
via the corresponding graphql query.
Please note that fetching stories is supported only when authentication
is enabled and the corresponding <username> is followed.
* [instagram] Add an only-matching test for stories
* [instagram] Simplify InstagramExtractor.items() and _extract_stories()
Simplify handling of typename in InstagramExtractor.items() and multi-line
string in _extract_stories(). NFCI.