Fix smartdown compare task

This task is used to compare a newly developed Smartdown flow against an existing/published ruby/YML version. It does this by running through the same set of inputs for both and comparing the results, and failing where there is a difference.

It had stopped working due to some interface changes in other parts of the code and was suffering from a lot of false positives.

- Transition flows now have '-transition' appended, by convention,
  which means the flows don't share the same slug. Update the helper
  to be aware of this and pick the non-transition flow to compare
- Remove some false positive diffs:
  - Exclude sidebar, breadcrumb and analytics content from diff, as
    only the non-smartdown flow will have a real sluf, with related
    content for those areas
  - hide the '-transition' URL differences
- Upgrade Smartdown to fix a previous questions page title bug, which was a blocker for releasing student fiance

I'll do a separate PR for publishing student finance, so we can use it as a reference of just the things that change when a smartdown flow transitions to published.