Add more placeholders to service desk custom templates

What does this MR do and why?

Add more placeholders for the Service custom templates.

Thank you email

• %{UNSUBSCRIBE_URL}: unsubscribe URL
• %{SYSTEM_HEADER}: system header message
• %{SYSTEM_FOOTER}: system footer message
• %{ADDITIONAL_TEXT}: custom additional text

New note email

• %{UNSUBSCRIBE_URL}: unsubscribe URL
• %{SYSTEM_HEADER}: system header message
• %{SYSTEM_FOOTER}: system footer message
• %{ADDITIONAL_TEXT}: custom additional text

Screenshots or screen recordings

Screenshots are required for UI changes and are strongly recommended for all other merge requests.

How to set up and validate locally

Unsubscribe URL

1. Create a customer service desk email template .gitlab/service_desk_templates/new_note.md with UNSUBSCRIBE_URL placeholder
2. Email the service desk to create an issue
3. Respond with a comment on the issue
4. Observe the 'unsubscribe' link in the response email

System header and footer

1. Create a customer service desk email template .gitlab/service_desk_templates/new_note.md with SYSTEM_HEADER and SYSTEM_FOOTER placeholders
2. Enable header and footer in emails
3. Email the service desk to create an issue
4. Respond with a comment on the issue
5. Observe the system header and footer in the response email

Custom additional text

1. Create a customer service desk email template .gitlab/service_desk_templates/new_note.md with ADDITIONAL_TEXT placeholder
2. Add custom additional text
3. Email the service desk to create an issue
4. Respond with a comment on the issue
5. Observe the custom additional text in the response email

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #285512 (closed)