Only display wallets for selected platform

Live preview: http://bitcointest1.us.to/en/choose-your-wallet

This change allows the page to scale better with the increasing number of wallets by displaying only selected wallets. This way the page can display more wallets on two lines instead of three, and prevent the education disclaimer to be buried too far at the end of the page. 

This also allows to avoid a layout with too much greyed out entries, which can be counter-intuitive, and avoid providing too much choice by default (to not fall too much into the paradox of choice). At some point, a "Show all" button could be added to display more than 12 wallets.

At the same time, this pull req fixes issue #488.