[FIX] web_editor: libsass warning and also incorrect / confusing @extend

Apparently libsass has never actually supported compound
selectors (or at least never the way people interpreted it should
work).

Therefore since libsass 3.5 (July 2017) that is deprecated and emits a
warning. Which is problematic because libsass is also hard-coded to
emit its deprecation warning to stderr[^1], resulting in a
few issues:

* because it's not decorated (let alone as a warning) it's not really
  noticeable on the runbot or in most dev logs
* but because it's not associated with a logger it's basically the
  only thing which shows up when running in `--log-level=warn` (as
  it's printed at least once per tour)
* and it's pretty gnarly to get the information back in Python as,
  again, hard-coded to stdout

odoo#70927 explored messing around with fds to intercept the
writes and re-emit them to the proper location, but that was
considered a bit too iffy.

And of course the intent was always to eventually fix the warning
itself, which qsm provided for in [a comment][fix-extend], the
important bit was the extension of `:disabled` so that's what's left
here to fix the warning. The logic of the fix is that the loader is
always used with the `form-control` class (internally) and the
`:disabled` part of the `form-control` styling is what we really want
to copy over. It's possible that third-party lose the styling if they
use `o_wysiwyg_loader` alone (without an explicit `form-control` next
to it) but that seems like an unlikely situation.

[^1] libsass 4 (sass/libsass#3135) surfaces warnings at the API level,
     and removes the hardcoded / direct stream writing

[fix-extend]: odoo#70927 (comment)

X-original-commit: 5c2c6b8

addons/web_editor/static/src/scss/web_editor.frontend.scss

@@ -5,7 +5,7 @@
 }
 
 .o_wysiwyg_loader {
-    @extend .form-control:disabled;
+    @extend :disabled;
     pointer-events: none;
     min-height: 100px;
     color: transparent;