Make version available in code (#282)

Token Introspection docs (#257)

It seems like there are multi ways to achieve that, all of them highliting
the importance of maintain a single source of truth for the version number.

More info: https://packaging.python.org/guides/single-sourcing-package-version/.

This PR includes one possible approach, open to discussion.

Added a new doc page related to Token Introspection Endpoint.
The documentation includes some introduction with links to the related RFCs and examples.

Enhancement: Increment RSA key size to 2048.

It seems like many lead institutions related with security are
recommending a minimum key length of 112-bits since 2013.
In order to achieve that, a RSA key size of 2048 (or more) is required.

Enhancement: AuthorizeView's static method strip-prompt-login moved

Enhancement: AuthorizeView's static method strip-prompt-login was moved to a new file oidc_provider/lib/utils/authorize.py in order to be more consistent with the implementation of other Views

Fix #283: bug in strip_prompt_login

AuthorizeView's static method strip_prompt_login fails to remove prompt login when other prompt values appear in the request.

Fixed #285 -- Ensured standard and custom claims can be used at the same time when using the OIDC_IDTOKEN_INCLUDE_CLAIMS setting

Revert "Fixed #285 -- Ensured standard and custom claims can be used at the same time when using the OIDC_IDTOKEN_INCLUDE_CLAIMS setting"

Revert "Fixed #285 -- Ensured standard and custom claims can be used at the same time when using the OIDC_IDTOKEN_INCLUDE_CLAIMS setting"

Fixed #285 -- Ensured standard and custom claims can be used at the same time when using the OIDC_IDTOKEN_INCLUDE_CLAIMS setting

Fixed #285 -- Ensured standard and custom claims can be used at the same time when using the OIDC_IDTOKEN_INCLUDE_CLAIMS setting

Django 2.1

Merge pull request #279 from ByteInternet/make_authorization_endpoint_and_client_more_easily_adjustable

Make it easier to change AuthorizeEndpoint and Client

Add missing space

To make it easier to change the AuthorizeEndpoint and Client we set them as class variables. Then people inheriting from the view are able to easily change them. In my personal case this helps with skipping consent more explicitly as defined in issue https://github.com/juanifioren/django-oidc-provider/issues/278

add natural key support to ResponseType

Have the option to use the more readable response type value rather than
the ResponseType id integer in fixtures and dumpdata output.

Prior to this change dumpdata represents response types like so:

    "response_types": [2]

And after this change when using `dumpdata --natural-foreign`:

    "response_types": [["code"]]

support multiple response types per client

The Dynamic Client Registration spec specifies multiple response_types
and grant_types per client
(https://openid.net/specs/openid-connect-registration-1_0.html#ClientMetadata).
Since grant_types can be inferred from response_types we should be able
to support both without needing to store grant_types.

This also helps with oidc-client-js which expects a client that supports
both "id_token" and "id_token token".