Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.
The actual em values were inconsistent with the docs,
while the docs were not the same as the comments:
* `sm` breakpoint was 34em (544px) not 480px.
* `lg` container max-width was 60rem (960px), less gutter than `md`.
Changed to 940px, same as Bootstrap 3.
* `xl` container max-width was 72.25rem which is 1140px not 1156px.
Changed to 1140px matching the comment but not the docs.
Addresses #17070 and #17388.
Mark Otto