Prevent 'Go' search button from overflowing

Sphinx designed the sidebar to be 210px large using 220px width and 5px
padding on each side. If the sidebar is smaller than that, the 'Go'
buttom will drop below the search input. Setting a padding larger than
5px on each side will indeed make the sidebar smaller due to border-box
being content-box by default in CSS.

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(If the 'Go' button is too close the body div, I can do something about that in another commit.)