Make all email templates customizable

Currently the user can customize enrollment emails and messages in the enrollment tab:

There are however more email templates that use the tera engine. Those templates can't be customized by the user.

```
mail_desktop_start.tera
mail_email_mfa_activation.tera
mail_email_mfa_code.tera
mail_enrollment_admin_notification.tera
mail_enrollment_start.tera
mail_enrollment_welcome.tera
mail_gateway_disconnected.tera
mail_gateway_reconnected.tera
mail_mfa_configured.tera
mail_new_device_added.tera
mail_new_device_login.tera
mail_new_device_ocid_login.tera
mail_password_reset_start.tera
mail_password_reset_success.tera
mail_support_data.tera
mail_test.tera
```

Consider exposing those templates in the admin UI and making them configurable.

Start by locating the existing enrollment email customization in the admin UI and the listed .tera templates, then trace how templates are loaded and stored. Done means all listed templates can be exposed in the admin UI and customized by users, with the existing enrollment behavior preserved.

Written by the indexing model from the issue text.