Fixathon: Web app dependency updates part 2 (Sass) (#29037)

* Update sass-loader@16.0.2

* Update sass part 1 (actually update it)

* Update scss files to remove at-charset and use at-use instead of at-import

at-charset hasn't been needed since we switched to dart-sass, and
at-import is now deprecated. Most of this was done using sass-migrator.

We still use at-import for files imported from node_modules, but sass
doesn't seem to complain about that

* Inline at-extend rule that was causing an error

* Remove or inline at-use rules in components package

webapp/channels/webpack.config.js

@@ -95,7 +95,7 @@ var config = {
                         loader: 'sass-loader',
                         options: {
                             sassOptions: {
-                                includePaths: ['src/sass'],
+                                loadPaths: ['src/sass'],
                             },
                         },
                     },