This commit transitions landing-page.css from the Django pipeline
to being compiled by webpack as landing-page.scss under the
'landing-page' and 'integration' bundles.
Apparently, essentially every one of our landing pages extending
portico.html had two copies of portico.css included in their head
section; one from porticocustomhead (or the super of customhead) and
the other directly included.
Clean this up by removing all these duplicate inclusions of the
portico stylesheet.