PLT-3143 Added serverside code for custom Emoji (#3311)

* Added model objects for emoji

* Added database tables for emoji

* Added settings for custom emoji

* Added serverside APIs and unit tests for custom emoji

* Added additional validation to catch duplicate emoji names earlier on

* Added additional validation to prevent users from adding emoji as another user

config/config.json

@@ -23,7 +23,9 @@
         "SessionCacheInMinutes": 10,
         "WebsocketSecurePort": 443,
         "WebsocketPort": 80,
-        "WebserverMode": "regular"
+        "WebserverMode": "regular",
+        "EnableCustomEmoji": true,
+        "RestrictCustomEmojiCreation": "all"
     },
     "TeamSettings": {
         "SiteName": "Mattermost",