www: Move theme into admin directory

What we're trying to do here is collect all of the non-"user" pages and
get them out of the root www tree. This makes it easier to determine
what files need to be considered for user testing, design work,
automated browser testing etc etc.
config.php: Add an admin links array

This adds an 'admin' element to the associative array return by
Pip_Links::val(), which can be used as such:
{{ _links.html.admin.theme }}.
Add a script to generate build summary

This bundles a bunch of useful information about the current website
build into a single variable in a generated PHP file.

Issue #177.
build/lib: Generate build-summary.php on make

Issue #177.
Add an admin index page

The idea here is that this can show useful information information about
the website, and links to admin tools.

Issue #177.
Merge branch 'issue/177'