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.
We started doing this for install docs in de2a2d0df, because `latest`
wasn't suitable and because I didn't know about readthedocs's `stable`
feature. The result has been that even with a checklist item, we
don't reliably update the link.
Instead, use the special `stable` version identifier on readthedocs to
link automatically to the highest version it knows about.
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.
Pointing these at the latest release, rather than the latest version
in master, allows us to make changes to the installer and document
them properly in master, without making the instructions confusingly
wrong for people who just go to the website or the GitHub repo page
and follow instructions to install.
This changes the second plans block label to dark grey responsively
when the width is less than 1390px because the white text does not
appear on that portion of the white background when the block collapses
down a line.