Fixes #20118: Add and use Sass variables to control the color of our embedded SVGs for custom form elements (#20700)

* Fixes #19784: Move from pixels to rems, add a margin-bottom: 0; to keep things aligned

* lintiong

- Fixes #20298 for vertical button group spacing
- Fixes #20784 for horizontal (default) button group spacing
- Alternate fix to #20823 which only applied a fix for vertical button groups

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Add Edge#9011176 bug (visual artifacts in scrollable dialogs)

https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/8282613/ has been marked as Fixed.
Refs #20335.

[skip sauce]

* Have same margin for <hN> as a <div> on .card-headers
Related #18726

When you use a `<hN>` as `.card-header` there is `margin-bottom: .5rem` added. The changes set the margin-bottom to 0
* Removed space

* Allow variables input-bg-focus and input-color-focus to be overridden.
* More consistency and control to customize shadows on .form-control:focus

* Fixes #19796: Remove mention of tooltip auto placement

* add note to migration docs

Update gruntfile to compile the Sass extras (flex, grid, and reboot) as part of the dist-css task (#20682)

h/t @bardiharborow for opening https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/15944476
Refs #20143
[skip sauce]

[skip sauce]

Merge branch 'v4-hamburger-fix2' of https://github.com/patrickhlauke/bootstrap into patrickhlauke-v4-hamburger-fix2

* Allow text of custom form labels to wrap like the default ones
* Switch from using after to force line breaks to float/clear combo in stacked custom forms

Use proper Sass variable interpolation to ensure the proper values are compiled and not the literal string of the variable name

* Fix docs for heading sizes (sizes here were from v3)
* Added rem sizes to header size docs
* Fixed erroneous px sizes

* Add .img-fluid to card images in columns to ensure proper resizing
* change that variable from regular value to variable
* use calc to figure out a tighter border for card images