add clearfix to #previousNextDoc, for small screens

[jrochkind]

On very small screens, on the Catalog#show, the floated "back to search"/"start over" escape their bounding box, and wind up below the bottom border, and too close to the next line:

Fixed simply by adding the .clearfix to the #previousNextDocument div:

Arguably, additional tweaks should be done, to keep these from being floated right on small screens, or even to hide them entirey on small screens (?), I'm not really sure, and the clearfix seems a good idea regardless, I think?

[cbeer]

+1 to the other styling suggestions. Want to do that first, or get this merged in as-is?

[jrochkind]

I'd go with merging this first -- I'm honestly not sure the best way to do the other stuff (both in terms of what it should look like, and how to implement it properly), so figured to just start with this to fix the worst problem.

[jrochkind]

I'm not sure at exactly what width-point the media query would do something different with those -- it should really do something different at the point the float 'wraps', but there's no way to do that with floats. Perhaps it should be using bootstrap grid columns instead of floats? You'd probably still need to change the text-align to left instead of right at some breakpoint, but at least you'd know the breakpoint that the bootstrap grid collapses.