Build: Add "pages" task to alias Assemble calls

Reuse the unminified Assemble files to create the minified versions instead of
re-running the task with a different assset path.
This is done by passing the unmin files through the HTMLCompressor and then
regexing the asset paths.
Merge pull request #5478 from nschonni/single-assemble-target

Build: Rework pages building