Moving openedx.core.lib.api to edx-drf-extensions

Rationale:

Currently edx-platform contains openedx.core.lib.api which are building blocks for creating APIs through DRF such as serializers, permission decorators and error handlers. These building blocks could be used across multiple projects at edX, ensuring that we are consistent across our APIs, but because they are in the edx-platform it makes their reuse difficult. I'm proposing we move all classes to the edx-drf-extensions repo, which already gets built as a package and installed into edx-platform.

File	Class / Method	Description	Status	Status Explanation
authentication.py	SessionAuthenticationAllowInactiveUser	Ensure that the user is logged in, but do not require the account to be active
	OAuth2AuthenticationAllowInactiveUser	This is a temporary workaround while the is_active field on the user is coupled with whether or not the user has verified ownership of their claimed email address
fields.py	ExpandableField	Field that can dynamically use a more detailed serializer based on a user-provided "expand" parameter
mixins.py	AbsoluteURLField	Field that serializes values to absolute URLs based on the current request
	PutAsCreateMixin	Backwards compatibility with Django Rest Framework v2, which allowed creation of a new resource using PUT
parsers.py	TypedFileUploadParser	Handles upload of files, ensuring that the media type is supported, and that the uploaded filename matches the Content-type
	MergePatchParser	Custom parser to be used with the "merge patch" implementation (https://tools.ietf.org/html/rfc7396).
permissions.py	ApiKeyHeaderPermission	Django REST Framework permissions class used to manage API Key integrations	NOT DOING	ApiKey is deprecated.
	ApiKeyHeaderPermissionIsAuthenticated	Allow someone to access the view if they have the API key OR they are authenticated	NOT DOING	ApiKey is deprecated.
	IsUserInUrl	Permission that checks to see if the request user matches the user in the URL	DONE	Moved to edx-drf-extensions
	IsCourseStaffInstructor	Permission to check that user is a course instructor or staff of a master course given a course object or the user is a coach of the course itself
	IsMasterCourseStaffInstructor	Permission to check that user is instructor or staff of the master course
	IsStaff	Permission that checks to see if the request user has is_staff access	DONE	Moved to edx-drf-extensions
	IsUserInUrlOrStaff	Permission that checks to see if the request user matches the user in the URL or has is_staff access	NOT DOING	Use rest_condition instead.
	IsStaffOrReadOnly	Permission that checks to see if the user is global or course staff, permitting only read-only access if they are not
	IsStaffOrOwner	Permission that allows access to admin users or the owner of an object. The owner is considered the User object represented by obj.user
serializers.py	CollapsedReferenceSerializer	Serializes arbitrary models in a collapsed format, with just an id and url
	CourseKeyField	Serializer field for a model CourseKey field	DONE	Moved to opaque-keys
test_utils.py	ApiTestCase	Parent test case for API workflow coverage
view_utils.py	DeveloperErrorViewMixin	A view mixin to handle common error cases other than validation failure (auth failure, method not allowed, etc.) by generating an error response conforming to our API conventions with a developer message
	ExpandableFieldViewMixin	A view mixin to add expansion information to the serializer context for later use by an ExpandableField
	view_auth_classes	Function and class decorator that abstracts the authentication and permission checks for api views.
	add_serializer_errors	Adds errors from serializer validation to field_errors. data is the original data to deserialize
	build_api_error	Build an error dict corresponding to edX API conventions
	RetrievePatchAPIView	Concrete view for retrieving and updating a model instance