Flask-micropub

Changed
- Added params to auth requests to support micropub.rocks tests

Added
- support JSON auth responses from token endpoints

Changed
- Allow all 2xx responses to indicate success.

Changed
- Support discovering endpoints from HTTP Link headers in addition
to searching the body of the page.

Changed
- Bugfix: authorization_handler was burning the auth code by
delegating to authentication_handler. This broke authorization for
endpoints that only allow codes to be used once. A side effect of
this is that authorization no longer falls back to authentication
when there is no token_endpoint or the token_endpoint request fails.

Changed
- Replace `next_url` parameter with more general `state`
(though we're keeping `next_url` for backward compatibility for now)

Changed
- Fix; fall back to indieauth.com when no authorization_endpoint is
specified (previous fix broke this).