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/flavours/glitch/styles/components/single_column.scss

@@ -94,7 +94,7 @@
   }
 }
 
-@media screen and (min-width: 600px) {
+@media screen and (width >= 600px) {
   .tabs-bar__link {
     span {
       display: inline;