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.

cmd/ursrv/serve/static/index.html

@@ -270,7 +270,7 @@ found in the LICENSE file.
             </h4>
           </div>
           <div id="collapseTwo" class="panel-collapse collapse">
-            <div class="panel-body">
+            <div class="panel-body less-padding">
               <div class="row">
                 <div class="col-md-6">
                   <table class="table table-striped">