Add search form on results page in Developer Resources

[morganestes]

When the search results page is displayed on Developer Resources, the only options to start a new search are to use the search form in the header for all of WordPress.org or to click the text "Developer Resources" (which is not clearly a link), click "Visit the Reference" button, go back to the main page, and search again.

Neither of these is ideal, so I think we should add the ability to search again from the results page.

Some options are to add a form to the top of the results page, similar to the main page; add the search icon that expands like on the individual reference pages, or add a simple search form that's more visible than the main "Search WordPress.org" search that's specific to the reference.

[morganestes]

Search results without search

[morganestes]

Search option on home page

[morganestes]

Mockup of search form on results page

[keesiemeijer]

Add search form on search result pages

[coffee2code]

In 2286:

developer.wordpress.org: Add search form to search results page.

Props keesiemeijer.
Fixes #1501.