[k7][UI Framework]: Figure out bottom bar padding issues.

[snide]

Bottom bar uses fixed position on the screen. This means it will often overlap content at the bottom of the normally rendered page. We should find some way to apply a bottom padding (bottom bar height + $kuiSizeL) to the bottom of the page to account for it.

Possibilities I can think of:

• Do a domNode check and just apply a class to the body. Only one body in a page, so should be ok.
• Make some sort of prop on a body component to account for this kind of thing. Lower level item would need to talk up the chain.

[snide]

Closed in favor of elastic/eui#49