Reduce repetition in Level 3 template for partner overrides.

As a step toward following 'Don't Repeat Yourself' for partner
site overrides, make the Level 3 results page use a series of
partial template includes, one per block. For any template that
is overridden for a partner site (or perhaps, for all), any
block that has something more involved than a single variable
probably ought to use an include. (#418)