Arrange HTML so that the search bar is part of the main element

[jcoyne]

Fixes #3252

[cbeer]

I'm not sure I understand the motivation for this change, but this seems like it could be incredibly disruptive to downstream applications are require a lot more overriding to recreate fairly common layout patterns:

[jcoyne]

I agree that this may be a challenge for adopters to take on. I believe this provides better semantics for accessibility. I believe this was raised by Darcy when looking at Geoblacklight (see #3252).

While implementing this it felt like our existing layouts (with content_for) made this a challenge to implement easily.

[dbranchini]

Let's close as won't do.