Sphinx 1.5 searchtools override has changed

[agjohnson]

Raised in readthedocs/readthedocs.org#2708

The problem here is that the Sphinx 1.5 searchtools.js_t has changed, but our override that removes the automatic execution has not been updated. This raises a few things:

• Should we drop support for Sphinx searchtools?
• The basic theme inserts this file inside the template, making overriding this pretty awful. We could override the searchtools.js_t with, effectively, a NOOP script. This would override whatever Sphinx 1.x adds to this file. The base implementation could be moved to our JS.
• The quickest solution is to instead just have two overrides, and the override is selected based on the Sphinx versioninfo.

Perhaps the first wave of fix here is to select the override based on Sphinx version, and then we just blow up support for the Sphinx searchtools on RTD altogether.