Use em for all text units, and a few style tweaks

Now things actually scale in a very nice way when people set their
browser font size. Which is good for accessibility, and more
generally just a polite thing to do.

Pixel measurements are also used for minimum width measurements for
the main content and footer section, so that they remain appropriate
even with very large fonts.

Style tweaks:
- Message area has appropriate padding
- Removed top padding on #container so page title starts a bit higher up
- Some unnecessary rules removed / merged
- Minor tweaks of a few sizes when switching to em

Change-Id: I103251a3cc62fdc8a325e6aca79b8b31c7ff5c10