Fix broken search functionality.

[mdickinson]

With Sphinx 1.8.5, documentation generated using this theme has a broken search field: searches fail with a JavaScript error resembling the following:

jQuery.Deferred exception: Search is not defined ReferenceError: Search is not defined
    at HTMLDocument.<anonymous> (file:///Users/mdickinson/Enthought/ETS/traits/docs/build/html/search.html?q=HasRequiredTraits&check_keywords=yes&area=default:33:25)
    at j (file:///Users/mdickinson/Enthought/ETS/traits/docs/build/html/_static/jquery.js:2:29999)
    at k (file:///Users/mdickinson/Enthought/ETS/traits/docs/build/html/_static/jquery.js:2:30313) undefined

This PR fixes the issue for me: in current Sphinx, search.html has a scripts template block which calls super() and then appends searchtools.js. But layout.html is missing the corresponding scripts template block.

@rkern Do you have bandwidth for a review?

[mdickinson]

Merging. @rkern: post-merge reviews also welcome! I suspect that at some point we're going to need to spend some time to bring layout.html up to date with upstream changes (both in SciPy's theme and in core Sphinx).