reorganize emojis and allow custom site emojis

images/emoji/custom/README

@@ -0,0 +1,17 @@
+# Custom site emojis
+
+Create a file called custom_emojis.json and add your custom emojis to this folder (or anywhere else you like).
+The content of custom_emojis.json should look as follows:
+
+´´´
+{
+    "some_emoji": {
+        "shortname": ":some_emoji:",
+        "filepath": "images/emoji/custom/some_emoji.png"
+    },
+    "another_emoji": {
+        "shortname": ":another_emoji:",
+        "filepath": "images/emoji/custom/another_emoji.png"
+    },
+}
+´´´