prefix oauth_callback path with site name

...so i can reuse my one instagram app that has a localhost oauth redirect URL. instagram only lets you set one redirect URL per app, and you only get 5 apps total, so i have to share one app for localhost testing across all of my apps, so they all need the same redirect URL. :/
drop instagram client id/secret usage since we get the access token from oauth-dropins

removes app engine dependency entirely when used as a library