Refactor pagination classes to avoid tag selectors & child selectors

 refactor blockqoutes to to avoid tag selectors & child selectors

Prevent anchors in tooltip example from splitting across lines

Card mixin

Use $input-border-width for both .form-control and .btn borders

Both widgets need to use the same border width because of input groups.
Thus:
$btn-border-width   => $input-btn-border-width
$input-border-width => $input-btn-border-width

[skip sauce]

Add entry about stateful buttons to v4 migration guide

Refs #18459, #17178
[skip sauce]

[skip sauce]

Add Wall of Browser Bugs entry related to #16028

https://bugs.webkit.org/show_bug.cgi?id=151933

[skip sauce]