Create new button styles for landing page components

[alexgibson]

A few different landing-page components feature new blue & black button styles:

#158
#157
#153

To save save work, we should create a separate PR for this new style. We should also make sure to update the newsletter component to use the new black button style:

https://protocol.mozilla.org/patterns/organisms/newsletter.html

[alexgibson]

/cc @michaelmccombie

[michaelmccombie]

The gradient for the download button should be

Default State
$color-green-50 to $color-green-70

Hover State
$color-green-60 to #color-green-80

Here's an image of the new download button style