* Update copyright range to include 2018

* Extend copyright date range on new files from 2017 to include 2018

Fixes #24311

Add `utilities/flex` to the dedicated grid Sass bundle so folks can take advantage of the utilities for basic flex alignment

- Correctly places the _custom.scss file first in the other two buids.
- Also removes the clearfix mixin from the grid build because flexbox yo.

- Creates new flexbox grid Sass file in our docs assets
- Updates the Gruntfile to compile said new Sass file and minify the output
- Update notice on flexbox docs page for how it works
- Only enable compiled flexbox grid CSS in hosted docs site, not in dev (for easier and specific debugging of all flexbox features)

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.