Update mobile button styles to match those specified on the FirefoxOS style guide.

I took the CSS from https://github.com/mozilla-b2g/gaia/blob/master/shared/style/buttons.css and made it fit into our existing layout. Visibly, the font weight is different, instead of 500 it is the default 400. This is so we do not have to add another font to the list of downloaded fonts. Otherwise, things should be pretty similar.

fixes #3635