[Piglit] Replace the HTMLIndex class in summary

[Dylan Baker]

This is easily the ugliest remaining part of the new summary code, this
series addresses this by first simplifying the class, and then moving
the necessary parts into the mako template.

Patches 1-3 lay groundwork for moving more logic into mako
Patch 4 moves some content into the mako templates that should have
        always been there
Patches 5-6 simplify the class, and then move that simplified class into
            mako
Patch 7 renames the templates to include their output extension