Fix SCSS linting issues (#2207)

* Disable font-family-no-missing-generic-family-keyword for font-awesome accessibility icons

* Run stylelint --fix

* Avoid `@extend` directives with doodle modal CSS

* Drop use of `@extend` for notification cleanup buttons SCSS

* Run prettier on SCSS

app/javascript/styles/mastodon/modal.scss

@@ -30,7 +30,7 @@
   }
 }
 
-@media screen and (max-width: 600px) {
+@media screen and (width <= 600px) {
   .account-header {
     margin-top: 0;
   }