Reduce calls to expensive methods in create_xblock_info

Improve load time of course outline page via `create_xblock_info()`. Several expensive methods are now only called when required (which, in this case, is when generating xblock info for the container page).

@cahrens @tymofij 

STUD-1997

Conflicts:
	cms/djangoapps/contentstore/views/component.py
	cms/djangoapps/contentstore/views/item.py
	cms/djangoapps/contentstore/views/tests/test_item.py