Stop using rems by default

# Stop using rems by default

Status: **Opened for visibility**
Reviewers: @jeffkamo @ry5n 

## Changes
1. Removed all references to the `rem()` function
1. Back to sweet sweet pixels

## Notes
- Some usage of this may result in half-pixels appearing in our `$margin` variables or line heights. I don't think this is a real problem but I'm open to opinions.