Add a pref for whether to display mention as icon or text

src/modules/config.js

@@ -70,6 +70,7 @@ export const defaultState = {
   useOneClickNsfw: false,
   useContainFit: false,
   greentext: undefined, // instance default
+  useAtIcon: undefined, // instance default
   hidePostStats: undefined, // instance default
   hideUserStats: undefined, // instance default
   virtualScrolling: undefined, // instance default

src/modules/instance.js

@@ -20,6 +20,7 @@ const defaultState = {
   background: '/static/aurora_borealis.jpg',
   collapseMessageWithSubject: false,
   greentext: false,
+  useAtIcon: false,
   hideFilteredStatuses: false,
   // bad name: actually hides posts of muted USERS
   hideMutedPosts: false,