Implement disabled-by-default setting for auto-replacement of plaintext emojis

FTR a list of plaintexts and their unicode equivalents can be found here - https://github.com/vector-im/riot-web/issues/4554#issuecomment-314374303

Pressing space after a matching emoji will replace the plaintext emoji with the equivalent unicode emoji.

src/components/structures/UserSettings.js

@@ -93,6 +93,10 @@ const SETTINGS_LABELS = [
         id: 'enableSyntaxHighlightLanguageDetection',
         label: 'Enable automatic language detection for syntax highlighting',
     },
+    {
+        id: 'MessageComposerInput.autoReplaceEmoji',
+        label: 'Automatically replace plain text Emoji',
+    },
 /*
     {
         id: 'useFixedWidthFont',