implement POST /emoji for apiV4 (#5868)

i18n/en.json

@@ -443,6 +443,10 @@
     "id": "api.command.init.debug",
     "translation": "Initializing command API routes"
   },
+  {
+    "id": "api.emoji.init.debug",
+    "translation": "Initializing emoji API routes"
+  },
   {
     "id": "api.command.invite_people.desc",
     "translation": "Send an email invite to your Mattermost team"