add filterText service method to strip HTML tags and Zalgo text

app/services/chat.js

@@ -8,7 +8,8 @@ const mongoose = require('mongoose');
 const ChatMessage = mongoose.model('ChatMessage');
 
 const ioEmitter = require('socket.io-emitter');
-
+const striptags = require('striptags');
+const unzalgo = require('unzalgo');
 
 const { SiteService } = require('../../lib/site-lib');
 
@@ -44,6 +45,17 @@ class ChatService extends SiteService {
       params: { messageId: message._id },
     });
   }
+
+  /**
+   * Filters an input string to remove "zalgo" text and to strip all HTML tags.
+   * This prevents cross-site scripting and the malicious destruction of text
+   * layouts.
+   * @param {String} content The text content to be filtered.
+   * @returns the filtered text
+   */
+  filterText (content) {
+    return striptags(unzalgo.clean(content.trim()));
+  }
 }
 
 module.exports = {