Contextual background-inverse text color fix

Migration docs: Fix typo in 'Reboot' about units

Update variable usage for input group addon

Instead of the regular border-radius var, use the input ones to match sizing with their associated controls.

Progress bar:fix use variable progress-border-radius instead of border-radius

Docs Tooltip Static Demo uses v3 class names

[skip sauce]
[skip validator]

v4: Enable flexbox CSS on flexbox grid page

- 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)

add Edge bug 8282613

- More details on columns and gutters
- Break it into clear sections

Remove mention of base class and fix grid examples

v4: Restore flexbox grid .col-{bp} class for auto layout

v4: Follow-up grid fixes