Remove all uses of selector & to support nested import

[glebm]

If we want to support namespaced import:

.twbs {
  @import 'bootstrap';
}

We cannot use selector &, see sass/sass#1817.

fieldset[disabled] & is not needed as we no longer support IE8.

[krnlde]

That is a very important decision! Thank you so much, I thought we're the only one namespacing our bootstrap.

[mischah]

👍

[cvrebert]

I'm totally in favor of removing fieldset[disabled] & since it's no longer necessary.

As for the general point, until Sass adds some feature (or folks discover some idiom/workaround) to aid that sort of prefixing, I think CSS postprocessing is still the way to go for that.
However, if the .btn .label change is really the only other required change, then I guess I'm okay with this.

[glebm]

We already do other things to support nested import, e.g. see https://github.com/twbs/bootstrap/blob/v4-dev/scss/_reboot.scss#L46, this is the only other change required as far as I can tell.

[chriseppstein]

If future versions need to use &, please note my comment here: sass/sass#1817 (comment)

[mdo]