Refine styles of menus, toasts, popovers, and modals (#12332)

* Refine styles of menus, toasts, popovers, and modals

This is a reintroduction of https://github.com/matrix-org/matrix-react-sdk/pull/12247, with the difference that modal styles have now been refreshed as well.

* Restore the fixed heights of some dialogs

* Fix formatting and flaky screenshot

res/css/views/settings/_IntegrationManager.pcss

@@ -15,16 +15,19 @@ limitations under the License.
 */
 
 .mx_IntegrationManager {
-    .mx_Dialog {
+    .mx_Dialog_border {
         box-sizing: border-box;
-        padding: 0;
         width: 60%;
         height: 70%;
-        overflow: hidden;
         max-width: initial;
         max-height: initial;
     }
 
+    .mx_Dialog {
+        padding: 0;
+        overflow: hidden;
+    }
+
     iframe {
         background-color: #fff;
         border: 0;