Oauth-dropins

Notable changes

* Bluesky:
* Normalize handles to lower case, remove leading `` ([bridgy1667](https://github.com/snarfed/bridgy/issues/1667)).

Miscellaneous changes in `webutil`.

Miscellaneous changes in `webutil`.

Notable changes

_Non-breaking changes:_

* IndieAuth:
* Store access token and refresh token in `IndieAuth` datastore entities.
* Misc `webutil` changes.

Breaking changes

* Remove `webutil.handlers`, which was based on the largely unmaintained [`webapp2`](https://github.com/GoogleCloudPlatform/webapp2). All known clients have migrated to [Flask](https://palletsprojects.com/p/flask/) and `webutil.flask_util`.
* Drop Python 3.6 support. Python 3.7 is now the minimum required version.

Non-breaking changes

* Add new `twitter_v2` module for Twitter's new [OAuth 2 with PKCE](https://developer.twitter.com/en/docs/authentication/oauth-2-0/authorization-code) support and [v2 API](https://developer.twitter.com/en/docs/twitter-api/migrate/whats-new).
* Blogger:
* Fix bug when user approves the OAuth prompt but has no Blogger blogs. Instead of crashing, we now redirect to the callback with `declined=True`, which is still wrong, but less bad.
* Mastodon:
* Change `MastodonAuth.access_token_str` from ndb `TextProperty` to `StringProperty` so that it's indexed in the Datastore.
* When the callback gets an invalid `state` parameter, return HTTP 400 instead of raising `JSONDecodeError`.
* Misc webutil updates.

Breaking changes

* Drop Python 3.5 support. Python 3.6 is now the minimum required version.

Non-breaking changes

* Switch from app_server to `flask run` for local development.
* Add `webutil.util.set_user_agent` to set `User-Agent` header to be sent with all HTTP requests.

_Breaking changes_

* Migrate from [webapp2](https://github.com/GoogleCloudPlatform/webapp2/) to [Flask](https://flask.palletsprojects.com/). webapp2 had a good run, but it's no longer actively developed, and Flask is one of the most widely adopted standalone web framework in the Python community.
* Remove `to()` class methods. Instead, now pass redirect paths to Flask's `as_view()` function, eg:

py
app = Flask()
app.add_url_rule('/start', view_func=twitter.Callback.as_view('start', '/oauth_callback'))

* Remove deprecated `blogger_v2` module alias.
* `webutil`: migrate webapp2 HTTP request handlers in the `handlers` module - `XrdOrJrdHandler`, `HostMetaHandler`, and `HostMetaXrdsHandler` - to Flask views in a new `flask_util` module.

Non-breaking changes

* `webutil`: implement [Webmention](https://webmention.net/) protocol in new `webmention` module.
* `webutil`: add misc Flask utilities and helpers in new `flask_util` module.