DevHub: No search form on 404 page

[SergeyBiryukov]

1. Go to ​https://developer.wordpress.org/non-existing/ (for example).
2. The page says: "It looks like nothing was found at this location. Maybe try a search?".
3. Ironically, there's no search form or breadcrumbs, like on other pages. There are two search forms in the HTML source code, but both are hidden with hide-if-js class.

Same for plugin and theme handbooks.

[SergeyBiryukov]

Related: #1739

[keesiemeijer]

Add searchform to 404 pages

[coffee2code]

In 4242:

developer.wordpress.org: Add search form to 404 pages.

Props keesiemeijer.
Fixes #2089.

[coffee2code]

@keesiemeijer: Thanks, as always, for the patch!

I committed the patch with a slight change: I still assigned the 'searchform-handbook' for the search form when appearing in a handbook and left that as the original exception for enabling autocomplete. This allowed the autocomplete to work in the 404 search form outside the handbook. This necessitated a CSS tweak to ensure the autocomplete's list didn't get clipped due to the rather short 404 page content space.