Added a simple XBlockMixin for courseware licenses

This allows course authors to choose between two difference licenses for their
course content: All Rights Reserved, or Creative Commons. In the backend, XBlocks
that wish to allow custom licenses need only inherit from LicenseMixin, which
adds a `license` field as a string.

License information is displayed in the Studio editor view, and just below the
rendered XBlock in the LMS. In addition, if the course block itself has a custom
license set, this license will display just below the main body of the page
on courseware pages.

This entire feature is gated behind the LICENSING feature flag.

cms/djangoapps/models/settings/course_details.py

@@ -42,6 +42,7 @@ class CourseDetails(object):
         self.overview = ""  # html to render as the overview
         self.intro_video = None  # a video pointer
         self.effort = None  # int hours/week
+        self.license = None
         self.course_image_name = ""
         self.course_image_asset_path = ""  # URL of the course image
         self.pre_requisite_courses = []  # pre-requisite courses
@@ -79,6 +80,7 @@ class CourseDetails(object):
         course_details.pre_requisite_courses = descriptor.pre_requisite_courses
         course_details.course_image_name = descriptor.course_image
         course_details.course_image_asset_path = course_image_url(descriptor)
+        course_details.license = getattr(descriptor, "license", None)
 
         for attribute in ABOUT_ATTRIBUTES:
             value = cls._fetch_about_attribute(course_key, attribute)
@@ -173,6 +175,10 @@ class CourseDetails(object):
             descriptor.pre_requisite_courses = jsondict['pre_requisite_courses']
             dirty = True
 
+        if 'license' in jsondict:
+            descriptor.license = jsondict['license']
+            dirty = True
+
         if dirty:
             module_store.update_item(descriptor, user.id)