Future-proof disabling of clicks on disabled `btn-link`. Fixes #25639
ensuring consistency between `btn-link` `<button>`s and `<a>`s when disabled.

* Prevent sign-in form from going outside viewport (#25388)

* Workaround IE centering bug

* #22600: prevent display:block for visible content

* allow transition for elements with show class

[Icons8](https://icons8.com/) is a great resource for icons.

They provide 15 different icon styles, exportable as fonts or svg

Minor edits for style and consistency

Fixes #25669, closes #25779.

Running `./build/change-version.js v4.0.0-alpha.6 v4.0.0` fixed this,
so the version change script works fine. I'm presuming instead this
change was just omitted from 35f80bb1, and then wouldn't have
been caught by subsequent runs of `change-version`, since it only
ever replaces the exact old version string specified.

See issue #25711

* load skippy markup from includes

This variable is not used anywhere.

Leftover from #18373.

As discussed in issue's comment:
https://git.io/vAuPW

Thanks!

This file no longer has intentional indentation errors

This adds `form-group` to add the same vertical space to the `form-check` as in other grouped controls
used in form sample markup.

Thanks!