gui: Replace global "Panel padding decrease" style with targeted class (#9659)

Transplanted from https://github.com/emlun/syncthing/pull/8 (meta-PR
into https://github.com/syncthing/syncthing/pull/9175) by request of
@acolomb (see:
https://github.com/emlun/syncthing/pull/8#discussion_r1724470574).

This padding decrease currently applies to _all_ collapsible panels, but
this padding decrease may not be appropriate for all collapsible panels.
In particular, it will not be appropriate for the collapsible panels
introduced in https://github.com/emlun/syncthing/pull/8.

gui/default/assets/css/overrides.css

@@ -304,11 +304,7 @@ a.toggler:hover {
     text-decoration: none;
 }
 
-/**
- * Panel padding decrease
- */
-
-.panel-collapse .panel-body {
+.panel-body.less-padding {
     padding: 5px;
 }