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

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Add entry about stateful buttons to v4 migration guide

Refs #18459, #17178
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Add Wall of Browser Bugs entry related to #16028

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

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Tweak naming and docs of font utility classes

* `.font-normal` is too generic. Rename it to `.font-weight-normal` for clarity.
* Rename `.font-bold` to `.font-weight-bold` so as to parallel `.font-weight-normal`.
* In docs, gloss "weight" term in relation to fonts for the benefit of non-typographiles.

Refs #18433

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Add .navbar-form entry to v4 migration docs

Erratum from 26da610b
Refs #18001

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Fix grammar of comment for <label> in Reboot

Add 3 more font-weight variables

Refs #14368

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Ports #18445 to v4.

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Add Wall of Browser Bugs entry for #15990

https://bugzilla.mozilla.org/show_bug.cgi?id=1139853
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Mention Mozilla Firefox meta-bug on Wall of Browser Bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1230801

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