[#1435] Make error response not redirectable when client is unauthorized

[#1426] Ensure ActiveRecord callbacks are executed on token revocation.

[#1407] Remove redundant and complex to support helpers froms tests (should_have_json, etc).

[#1416] Don't add introspection route if token introspection completely disabled.

[#1410] Properly memoize current_resource_owner value (consider nil and false values).

[#1415] Ignore PKCE params for non-PKCE grants.

[#1418] Add ability to register custom OAuth Grant Flows.

[#1420] Require client authentication for Resource Owner Password Grant as stated in OAuth RFC.

[IMPORTANT] you need to create a new OAuth client (Doorkeeper::Application) if yoo didn't
have it before and use client credentials in HTTP Basic auth if you previously used this grant
flow without client authentication. For migration purposes you could enable
skip_client_authentication_for_password_grant configuration option to true, but such behavior
(as well as configuration option) would be completely removed in a future version of Doorkeeper.
All the users of your provider application now need to include client credentials when they use
this grant flow.

[#1421] Add Resource Owner instance to authorization hook context for custom_access_token_expires_in
configuration option to allow resource owner based Access Tokens TTL.

[#1371] Add #as_json method and attributes serialization restriction for Application model.
Fixes information disclosure vulnerability (CVE-2020-10187).

[IMPORTANT] you need to re-implement #as_json method for Doorkeeper Application model
if you previously used #to_json serialization with custom options or attributes or rely on
JSON response from /oauth/applications.json or /oauth/authorized_applications.json. This change
is a breaking change which restricts serialized attributes to a very small set of columns.

[#1395] Fix NameError: uninitialized constant Doorkeeper::AccessToken for Rake tasks.

[#1397] Add as: :doorkeeper_application on Doorkeeper application form in order to support
custom configured application model.

[#1400] Correctly yield the application instance to allow_grant_flow_for_client? config
option (fixes #1398).

[#1402] Handle trying authorization with client credentials.