'Return whether the block has been bookmarked by the user.
Arguments:
usage_key: UsageKey of the block.
Returns:
Bool'

def is_bookmarked(self, usage_key):

    usage_id = unicode(usage_key)
    bookmarks_cache = self._bookmarks_cache(usage_key.course_key, fetch=True)
    for bookmark in bookmarks_cache:
        if (bookmark['usage_id'] == usage_id):
            return True
    return False
