app_variable: Refactor a settings-nested-checkbox-vertical-margin.

This refactors a CSS variable for checkbox field margin for checkboxes
nested in another field. For example, the "integration events" options
in the "Generate integration URL" modal. Those nested checkboxes are
spaced out closer together.

This is a prep commit for #34524, that PR will add another nested
checkbox in the "Add a new bot" modal.

web/styles/app_variables.css

@@ -2059,6 +2059,8 @@
     );
     /* The SVG must be adjusted when changing the border color above. */
     --svg-url-rendered-checkbox: url("../images/checkbox.svg");
+    /* Nested checkboxes in settings menu */
+    --settings-nested-checkbox-vertical-margins: 0.3125em 0; /* 5px at 16px */
 
     /* Theme-dependent SVGs */
     --svg-url-thumbnail-loader: url("../images/loading/loader-black.svg");