Fix left blocker weirdness

Firstly, we weren't ever returning `true`.

The other issue was that if the intersected element was a left blocker but it extended below the bottom of the injected unit, we weren't identifying it as a blocker.

I also added `<ol>` tags as left blockers.

See: https://github.com/GigaOM/gigaom/issues/5781 and https://github.com/GigaOM/gigaom/issues/5772