implement a simple caching system for themes 3

src/components/settings_modal/tabs/general_tab.vue

@@ -200,6 +200,14 @@
     <div class="setting-item">
       <h2>{{ $t('settings.post_look_feel') }}</h2>
       <ul class="setting-list">
+        <li>
+          <BooleanSetting
+            path="forceThemeRecompilation"
+            :expert="1"
+          >
+            {{ $t('settings.force_theme_recompilation_debug') }}
+          </BooleanSetting>
+        </li>
         <li>
           <ChoiceSetting
             id="conversationDisplay"