- remove screen reader stuff in favor of explicit labels
- remove 4 and 5 items to keep examples simpler

frees us up to not have to encourage so much markup throughout these pagination examples

this, coupled with the move to flex, fixes #20029

- Overhauls the states, including the link/button variants, for list groups to better match how we handle .btn states.
- Moved the .list-group-item-action styles before the .list-group-item so that we don't have to do as much overriding.
- Removed the plain-hover-focus mixins from the disabled and active states since they're unnecessary.
- Added support for :active states on the .list-group-item-action variant (for the current click state).
- Removed the heading and text classes and variables since we can accomplish the same thing with utilities.
- Added support for :disabled on the .list-group-item-action as well since we can use those with button elements.
- Rearranged docs to reflect all the above changes.
- Reformatted some Sass variables.

- 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.

- No need to set hover and focus on disabled or active elements. Disabled has no interactivity and active will inherit the focus.
- Also removes two now unused vars.

don't implement for the outline style because outline buttons get no shadow by default

also be sure to apply it for the active state when shadows are enabled

shoutout https://medium.com/simple-human/buttons-shouldnt-have-a-hand-cursor-b11e99ca374b#.riy12ck0m

- removes original outline removal (hah)
- replaces it with an explicit `outline: 0` on `.btn`
- instead of replicating `:hover` for `:focus`, uses custom and themed `box-shadow` for an "outline"
- not mapped to the `$enable-shadows` variable because accessibility

Trying to simplify our output here by revamping these selectors. We overcomplicated things by setting hover styles for nearly every state (disabled and active included), and we set them in the wrong order.

This commit does the following:

- Reorders states so disabled comes before active, thereby removing the need to set disabled-active styles.

- Removes all focus and hover styles from disabled states as those will naturally inherit from the default button state.

- Renamed `.open` to `.show` to fix dropdown toggle highlighting.

- Tweaked some indendation in the Sass.

shoutout to @Starsam80 for the fix