feat(security): add server-side email HTML sanitizer utility

Add Symfony HTML sanitization support and introduce a reusable
EmailHtmlSanitizer utility for email-safe rich text rendering.

- add dependency: symfony/html-sanitizer (^7.4)
- add EmailHtmlSanitizer class under lib/Common/Security
- decode legacy HTML entities before sanitizing
- allow basic formatting tags and safe link schemes

If needed in the future could use the symfony/html-sanitizer for other
tasks.

Author: JohnVillalovos

composer.json

@@ -30,7 +30,8 @@
         "phpmailer/phpmailer": "^7.0",
         "claviska/simpleimage": "^4.2",
         "mashape/unirest-php": "^3.0",
-        "vlucas/phpdotenv": "^5.6"
+        "vlucas/phpdotenv": "^5.6",
+        "symfony/html-sanitizer": "^7.4"
     },
     "suggest": {
         "pear/net_ldap2": "Required for the LDAP authentication plugin. Install with: composer require pear/net_ldap2"