unified layout-setting code and made an option to control or disable

third column behavior

src/components/settings_modal/tabs/general_tab.js

@@ -38,6 +38,11 @@ const GeneralTab = {
         value: mode,
         label: this.$t(`settings.mention_link_display_${mode}`)
       })),
+      thirdColumnModeOptions: ['none', 'notifications', 'postform'].map(mode => ({
+        key: mode,
+        value: mode,
+        label: this.$t(`settings.conversation_display_${mode}`)
+      })),
       loopSilentAvailable:
       // Firefox
       Object.getOwnPropertyDescriptor(HTMLVideoElement.prototype, 'mozHasAudio') ||