Flask-dance

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Added Digital Ocean pre-set configuration

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* Updated Discord to use the new discord.com instead of the old discordapp.com
* Add Strava pre-set configuration

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* Updated Meetup and Nylas pre-set configurations
to include the ``client_id`` in the OAuth token request.
* Removed Okta pre-set configuration, since it doesn't add any value over
using ``OAuth2ConsumerBlueprint`` directly.
* Updated Azure to allow defining ``authorization_url_params``

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* Added Heroku pre-set configuration

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* Flask-Dance now provides a ``betamax_record_flask_dance`` testing fixture,
for recording and replaying HTTP requests using Betamax_. See the testing
documentation for more information.
* Added LinkedIn pre-set configuration

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Changed (**backwards incompatible**)
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* The backwards-compatible references to "backend" have been removed.
Use "storage" instead.
* The columns defined in ``OAuthConsumerMixin`` now set ``nullable=False``.
If you are using the SQLAlchemy storage and are upgrading from a previous
version of Flask-Dance, you may want to do a database migration.
* Previously, Flask-Dance had an undocumented
feature where it would automatically redirect based on a ``next``
parameter in the URL. This undocumented feature has been removed.
* All pre-set configurations now use a consistent
naming scheme for pulling client IDs and client secrets from the app config.
The following configurations have changed: Dropbox, Meetup, Twitter,
and Zoho.
* Replace ``lazy`` dependency with
`werkzeug.utils.cached_property <http://werkzeug.pocoo.org/docs/0.14/utils/#werkzeug.utils.cached_property>`__