Add support for link content type

lti_consumer/lti_1p3/constants.py

lti_consumer/lti_1p3/extensions/rest_framework/constants.py

...consumer/lti_1p3/extensions/rest_framework/serializers.py

lti_consumer/lti_1p3/tests/test_consumer.py

lti_consumer/plugin/views.py

lti_consumer/templates/html/lti-dl/render_dl_content.html

lti_consumer/templates/html/lti-dl/render_link.html

...consumer/tests/unit/plugin/test_views_lti_deep_linking.py