When searching a handbook, only search the handbook

[johnbillion]

Handbooks show a search field above their Chapters menu. When searching from this field, one would assume that it searches the handbook, when in fact it searches the whole site and often returns irrelevant results such as a blog posts.

This search field should only search the handbook post type. Its form action URL should probably be changed to something like home_url('handbook').

[coffee2code]

In 5237:

Breathe: Constrain handbook searches to handbook pages.

Introduces searchform.php template.

Fixes #2623.