Allow a custom gutter to be specified when creating columns

WEB-20590
Use pixels for column widths so nesting works

- Modify bootstrap grid-framework to output px widths if it's
  provided with a container width
- Add nesting examples to layout style page
- Add more descriptive variable names for gutter widths

WEB-20775
Move common Bootstrap column css into a single mixin

WEB-20775
Calculate column widths in one place

Create a mixin to calculate column widths, and use it for both
the grid classes and grid mixins.

WEB-20775
Use pixels for grid mixin column widths

WEB-20775
Don't break compatibility of Bootstrap's grid mixins

- Don't depend on values set in our variable_overrides
- Allow the container width to be passed in, however default to
  the old behaviour of percentage width columns

WEB-20775