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/index.html

@@ -413,7 +413,7 @@
                 </h4>
               </button>
               <div id="folder-{{$index}}" class="panel-collapse collapse">
-                <div class="panel-body">
+                <div class="panel-body less-padding">
                   <table class="table table-condensed table-striped table-auto">
                     <tbody>
                       <tr class="visible-xs">
@@ -681,7 +681,7 @@
               </h4>
             </button>
             <div id="device-this" class="panel-collapse collapse in">
-              <div class="panel-body">
+              <div class="panel-body less-padding">
                 <table class="table table-condensed table-striped table-auto">
                   <tbody>
                     <tr>
@@ -796,7 +796,7 @@
                 </h4>
               </button>
               <div id="device-{{$index}}" class="panel-collapse collapse">
-                <div class="panel-body">
+                <div class="panel-body less-padding">
                   <table class="table table-condensed table-striped table-auto">
                     <tbody>
                       <tr class="visible-xs">