Revamp grid classes to require .col base class and improve flexbox grid classes with auto layout via flex: 1

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.

Add migration guidance regarding vendor prefix mixins

Remove section on pager changes from migration, since pagers have been removed entirely

Add Wall of Browser Bugs entry for https://webkit.org/b/153856

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

Revert separation of <hN> and .hN heading styles

This reverts commit deeb74e3.

Reverts #18038
Fixes #18992
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Mention https://webkit.org/b/153852 in browser compatibility docs

Refs #19077
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Add Wall of Browser Bugs entry for https://webkit.org/b/153852

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)

Update shrinkwrap

Closes #19049