Strip out .err from our syntax highlighting styles because it fucks up our placeholders in code snippets

- Flexbox responsive behavior fixed with specific .col-{breakpoint} classes now added
- Dropped the make-col mixin in favor of a column-basics placeholder that we can extend across our grid infrastructure
- Updated docs to use required .col-xs-12 (as a safeguard for when folks enable flexbox mode--this isn't necessary in default grid mode)
- Update flexbox grid docs to include responsive docs, tweak some other bits too

flip things around again, no more base class, try out some new stuff to keep responsive flex grid working

Capture the slug once before applying it and add a slash to it to so we get a unique string to match against instead of a fuzzy partial 'contains'. Helps avoid 'grid' highlighting 'flexbox-grid', for example.

Refs https://github.com/twbs/bootstrap/issues/14839#issuecomment-60940738
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Refs #19080

Refs #19077
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Refs http://getbootstrap.com/getting-started/#overflow-and-scrolling
Refs #14839
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Dump the type margin overrides in alerts for utils, document that particular example, sidesteps part of #18884

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

Closes #19049