stream_settings: Fix save/discard widget on narrow screens.

In narrow screens, the save/discard widget was incorrectly shown
even with no changes, due to the CSS intended to switch it from `inline-block` 
to `block display.

Fixed by adding `display: none` to `.hide` class for @media(width < 575px)
to hide buttons on screen width less than 575px.

Fixes #24589.

web/styles/subscriptions.css

@@ -1165,6 +1165,10 @@ div.settings-radio-input-parent {
         .save-button-controls {
             display: block;
             margin: 0 0 10px;
+
+            &.hide {
+                display: none;
+            }
         }
     }
 }