Draft: Hide search when docs are shipped with Omnibus

As part of gitlab#214164 (closed) and omnibus-gitlab!4726 (closed), this implements a way to hide the search when the docs site is shipped with Omnibus.

Closes #1071 (closed)

Test locally:

NANOC_ENV=omnibus bundle exec nanoc live

before	after

Missing from this MR:

• We can probably refine the CSS "Explore all topics" of the landing page, but that's for another MR.
• /search/ is still there. I couldn't figure a way to omit building a whole page. Possibly we can do something in Rules.