Fix footer overlapping issue

This sets a minimum height of 100vh (100% viewport height) on the content wrapper.

The result is a content area that is always at least 100% the height of the viewable area regardless of the amount of content.

Pros:

• Aligns with how the menu behaves in the product (menu is always 100% of the viewport height.
• More real estate for the menu items in the viewable area (fewer menu items are hidden).
• Content height is more predictable so we'll never end up with a global nav that scrolls behind the footer.

Cons:

• We'll have 'wasted' whitespace in the content area on pages where there is not a lot of content.

Preview

before	after

Closes #466 (closed)