Calculate has_changes only Section/Subsection only on Outline page

This fixes a bit of overhead introduced in https://github.com/edx/edx-platform/pull/5020
`@memoize_request_cache` still improves the result by couple of queries.

@cahrens @polesye @olmar 

Number of queries for `test_get_query_count(branching_factor, chapter_queries, section_queries, unit_queries, problem_queries)`

Previously:

    (1, 21, 23, 35, 37),
    (2, 22, 24, 38, 39),
    (3, 23, 25, 41, 41),

without memoize:

    (1, 14, 16, 28, 30),
    (2, 15, 17, 39, 32),
    (3, 16, 18, 52, 34),

with memoize:

    (1, 14, 16, 28, 30),
    (2, 15, 17, 35, 32),
    (3, 16, 18, 46, 34),

I propose we leave memoize in, unless RAM usage becomes an issue.