[Web] Optional HTML in system mails (#3879)

* HTML in system mails

* Update functions.inc.php

* Update functions.inc.php

Co-authored-by: André Peters <andre.peters@debinux.de>

data/web/lang/lang.en.json

@@ -139,6 +139,7 @@
         "ban_list_info": "See a list of banned IPs below: <b>network (remaining ban time) - [actions]</b>.<br />IPs queued to be unbanned will be removed from the active ban list within a few seconds.<br />Red labels indicate active permanent bans by blacklisting.",
         "change_logo": "Change logo",
         "configuration": "Configuration",
+        "convert_html_to_text": "Convert HTML to plain text",
         "credentials_transport_warning": "<b>Warning</b>: Adding a new transport map entry will update the credentials for all entries with a matching nexthop column.",
         "customer_id": "Customer ID",
         "customize": "Customize",
@@ -190,6 +191,7 @@
         "hash_remove_info": "Removing a ratelimit hash (if still existing) will reset its counter completely.<br>\r\n  Each hash is indicated by an individual color.",
         "help_text": "Override help text below login mask (HTML allowed)",
         "host": "Host",
+        "html": "HTML",
         "import": "Import",
         "import_private_key": "Import private key",
         "in_use_by": "In use by",
@@ -218,6 +220,7 @@
         "oauth2_redirect_uri": "Redirect URI",
         "oauth2_renew_secret": "Generate new client secret",
         "oauth2_revoke_tokens": "Revoke all client tokens",
+        "optional": "optional",
         "password": "Password",
         "password_repeat": "Confirmation password (repeat)",
         "priority": "Priority",