Revert "Remove SearchPathSetter"

This reverts commit 22cd51aa80a43aea391a92cfd92ad1022c7db5a2.

Service Manual needs custom search to look in a particular search index.

My intention is to make this change to slimmer, release a new version and also make
a change to designprinciples application or nginx to add the necessary header for
service-manual content.

@jystewart, does that sound reasonable?