Create a sticky purchase bar

JSFiddle as an example: https://jsfiddle.net/tj111wzr/

This is kind of a few parts:
1. Change the pay-what-you-want section to behave more like the humble-button widget in Appcenter with the popover dropdown
2. Stick it to the page so that any time you're reading the home page you can start the download
3. Remove the second call to action at the bottom of the page since this covers that functionality

Humble button for reference:

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