Make section subtree add-options depend on SectionBase.

Per [68905860](https://www.pivotaltracker.com/story/show/68905860).  Problem was traced to tree footer basing expander buttons on whether it's a `Section` object instead of `SectionBase`.

Also, replaced a conditional "Section/Clause" display with with a display of the class name, as the Section vs Clause display should now no longer depend on parent type.

Note: Debugging difficulty arose from same branch doing different things on DEV server vs localhost; fixed by re-running `build_compass`.