Apply layers to slide-down interpretations in sectional view

Slide-down interpretations didn't have layers applied to them in a sectional view, as the only data that was requested was for the regtext (not the interpretation). This isn't a problem when viewing the full reg since the full reg pulls in all layer info.

To solve this,
* Interpretation layer delegates to partial view for rendering
* Apply cacheing to layer information; grab it once for each regulation, version, layer_name tuple

This change costs us a wee bit of functionality as it drops selectable layers applying to the slide-down interpretation. If that seems like a big deal, we can return to passing this information in to the layer upon construction, it'd just take a bit more time.