PLT-2816 Fixed handling of Unicode 8 emojis (#2924)

* Updated twemoji to properly recognize Unicode 8.0 emojis

* Updated unicode emoji parser to only render emojis we support as images

* Corrected filename for South African flag emoji

* Added Mattermost emoticons!

* Added additional emoticons to test files

webapp/utils/emoji.json

@@ -8181,6 +8181,14 @@
   , "tags": [
     ]
   }
+, {
+    "aliases": [
+      "mm",
+      "mattermost"
+    ]
+  , "tags": [
+    ]
+  }
 , {
     "aliases": [
       "basecamp"