Fix/security extra message wrapping

[stgmsa]

Description

This fixes issue #8192,
In security event configuration, if admin wish to use a variable in additional email message, it will be ignored during template parsing stage during the config loading stage. This is a fix to avoid this.

Impacts

changed default behavior of using variables in extra message in security event email template.
In the admin UI, admins still use [% variable_name %] to wrap the variable, but in the backend, we use another layer to wrap the variable once more to avoid this to be parsed to null when loading config files.

Issue

fixes #8192

Delete branch after merge

YES

Checklist

• [yes] Document the feature
• [no] Add OpenAPI specification
• [no] Add unit tests
• [no] Add acceptance tests (TestLink)

Bug Fixes

fixes the issue that a variable is not parsed in extra message in security event's template. #8192