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.