Multiple Ui fixes (#6272)

* PLT-6453 - Improving headings in the web app

* PLT-6414 - Fixing view members modal issue

* PLT-5863 - Fixing alignment in new channel menu

* PLT-6391 - Fixing code block overlay

webapp/sass/components/_modal.scss

@@ -275,6 +275,10 @@
                 overflow-y: visible;
             }
 
+            .post-code__container {
+                overflow: auto;
+            }
+
             .modal-image__wrapper {
                 @include border-radius(3px);
                 display: table-cell;