Use underlying search.html file.

@russell -- I'm assigning this PR to you because your commits added `search.html` customizations to SBT in the first place, and I want to make sure I'm not stomping on something aside from the UI button that I don't mean to.

**Background**: Search doesn't work locally for `file://`-based URLs in at least Chrome because of access controls. Sphinx 1.2 added a hack to get some, but not all, search functionality added to local searching.

@fjfeijoo opened issue #82 and pull request #83 to add in the script tag extras for Sphinx 1.2.

**This PR**: My thought was to instead just remove `search.html` from our theme customizations and use the underlying one, whatever it is.

Here is a screenshot of existing live site:

![screen shot 2014-01-27 at 8 25 04 am](https://f.cloud.github.com/assets/445557/2010566/9bae2352-876f-11e3-998b-366956a4b31e.png)

And, here is local `file://`-based with this PR applied, using Sphinx 1.2

![screen shot 2014-01-27 at 8 25 37 am](https://f.cloud.github.com/assets/445557/2010569/ac79dbd6-876f-11e3-899f-2e095b2aae58.png)

There are two main things to note here:

1. We don't get local file highlights in local `file://`-based search even with the PR due to Chrome access controls. This is orthogonal to the actual PR here, but just pointing it out.
2. The search button UI is default submit button.

So, what's the consensus here?

I like the approach in this PR over us manually tracking changes because we just get whatever Sphinx has by default, but I'm willing to entertain other ideas too... Thanks!