Split conversation display style into two different settings

linear => linear (now default)
simple_tree => tree / conversationTreeAdvanced=false
tree => tree / conversationTreeAdvanced=true

src/components/settings_modal/tabs/general_tab.vue

@@ -165,6 +165,11 @@
           v-if="conversationDisplay !== 'linear'"
           class="setting-list suboptions"
         >
+          <li>
+            <BooleanSetting path="conversationTreeAdvanced">
+              {{ $t('settings.tree_advanced') }}
+            </BooleanSetting>
+          </li>
           <li>
             <label for="maxDepthInThread">
               {{ $t('settings.max_depth_in_thread') }}