Implement right sidebar option as user option

Currently an admin can set the "sidebarRight" option, but the user is unable to change it for themselves.

This option felt like a natural fit, and I currently use it already on my instance.

src/components/settings_modal/tabs/general_tab.vue

@@ -11,6 +11,11 @@
             {{ $t('settings.hide_isp') }}
           </BooleanSetting>
         </li>
+        <li>
+          <BooleanSetting path="sidebarRight">
+            {{ $t('settings.right_sidebar') }}
+          </BooleanSetting>
+        </li>
         <li v-if="instanceWallpaperUsed">
           <BooleanSetting path="hideInstanceWallpaper">
             {{ $t('settings.hide_wallpaper') }}