Improved download button and features grid

Rather than having multiple download buttons and then hiding one of
them, the page now just contains one button, the properties of which are
modified according to the OS being used. If JS is disabled, the download
button will point to the 'Downloads' page.

The features grid used to look a bit odd, it has now been fixed.
Added Downloads page & Decoupled the less file

A 'Downloads' page has been added which contains all available
downloads.The download links are listed in a table, so that in future
we can easily add a new row for a new version etc.

The design of the download page has been taken from:
https://www.apachefriends.org/download.html

The 'styles.less' has been broken into multiple files for easier
maintenance.