[instagram] rewrite

(#1113, #1122, #1128, #1130, #1149)

Rely on the results of GraphQL queries instead of requesting data
for each post separately via '/p/<shortcode>/?__a=1'.

This might result in some missing metadata, and there might be some
issues for '/channel/' and '/saved/' URLs, but at least downloading
from the regular post listings should work without issues and without
getting users blocked/banned.

TODO: reimplement support for stories