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.
Armaan Ahluwalia
Tim Abbott
Akash Nimare
Brock Whittaker