Moves IE styles to their own override

Moves IE styles to their own stylesheet that appears last in the
sequence and is used to override the base styles with IE-specific
quirks. This allows:
- Removal of :after and :before selectors from the main stylesheet.
- Easier removal of legacy IE support, should that be removed in the
future.
Merge pull request #586 from codeforamerica/ie-style-separation

Moves IE styles to their own override