fixes #17573

remove magic dropdown positioning for fixed bottom; folks should set this as needed with .dropup modifier

- Removes the plain-hover-focus mixin from active and disabled states; no need for them.
- Adds :active and :disabled since we can use button elements here, too.
- Wrap the disabled background-image override in an -gradients condition.

Websites might use different weights such as medium or thin or use light as default weight instead of normal.
Also referenced: #18605 and #16170.

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

* lintiong

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- 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)

- New vars
- New function for stripping units so we can combine rems and ems in math functions
- Add new classes for sizing and spacing around the split dropdown toggle so that the caret isn't misaligned

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Fix #17276

Properties should be ordered border-top, border-bottom

Fix #17052

Fix #17144

Fix #17135

- Remove nested > selectors for dropdown items
- Update docs examples to use all the new markup