fix(Markdown): render markdown in more secure way

This commit changes markdown sanitization behaviour in following way:

class, style and data-* attributes are removed by default. These attributes
open possible vulnerability vectors to attackers.

The original behavior of sanitizer (before this commit) can be enabled by *useUnsafeMarkdown* configuration option.
Use this configuration option with caution and only in cases when you know
what you're doing.

docker/configurator/variables.js

@@ -71,6 +71,10 @@ const standardVariables = {
     type: "boolean",
     name: "showCommonExtensions"
   },
+  USE_UNSAFE_MARKDOWN: {
+    type: "boolean",
+    name: "useUnsafeMarkdown"
+  },
   OAUTH2_REDIRECT_URL: {
     type: "string",
     name: "oauth2RedirectUrl"