* Initial spike of consolidated form checks

* Stub out forms rearrangement

- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS

* Move input group Sass file to forms subdir

* Start to split and move the docs around

* Simpler imports

* Copyediting

* delete overview file

* Remove commented out code

* remove the custom-forms import

* rewrite flex-check as form-check, replace all custom properties

* Remove old forms doc

* stub out new subpage link section

* update migration guide

* Update nav, forms overview in page nav, and descriptions

* fix check bg position

* fix margin-top calculation

* rename .custom-select to .form-select

* Update validation styles for new checks

* add some vertical margin, fix inline checks

* fix docs examples...

- Remove IE10 background hack
- Ability to change the pseudo element
- Ability to change the z-index
- Remove pointer events, just inherit this

The cross icon was confusing since some browsers use it as a reset button.

Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.

* Use $popover-border-radius

* Move and rename the local $offset-border-width to the global variable

- Line height & font family from html were directly overridden by body, so they were redundant thus removed
- The remaining properties are moved to body
- Shim for IE10 tags removed
- hr cleanup
  - Combined styles from reboot & type
  - Changed from borders to background
    - This allows us to keep the color inherited.
    - Opacity can be used to change the intensity
    - Size attribute will not change the height instead of making the hr look like an input field
  - Better variable names
- headings cleanup
  - Moved all element styling to reboot
  - Type will now extend the element styling. This way we will save some redundant code
- small & mark are also moved to reboot. type does not contain any element styling anymore
- font weight removed from small, this did not really made sense

Currently, `$toast-border-radius` uses the value of `.25rem`. This is inconsistent with other variables such as `$btn-border-radius` and `$input-border-radius`, which uses the "global" `$border-radius` variable.

- Drop hover and focus states
- Drop .badge-* variants and associated mixin for .bg-* utilities
- Drop .badge-pill for .rounded-pill utliity and adjust horizontal padding to find middle ground of .badge and old .badge-pill
- Remove unused variables
- Add default color value to .badge base class
- Update docs to reflect changes

* Add map-name param to _assert-starts-at-zero

* Set default value for backwards compatibility

In a scenario where `$input-line-height` or `$input-font-size` are different than `$input-btn-line-height` and `$input-btn-font-size` the final `$input-height-inner` was wrongly calculated.