ServiceDesk: Added footer and header messages to outgoing emails

What does this MR do and why?

This changes allow to add footer and header messages to outgoing emails from Service Desk.

Solves add Footer Message to outgoing emails from Serv... (#344819 - closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set a Header(and/or Footer) Message in

• Admin > Settings > Appearance > System header and footer > Header Message (and/or Footer Message)

2. Check

• [ x ] Enable header and footer in emails

3. Send any email to ServiceDesk email address.
4. Get "thank you" email with correct header and/or footer.

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 #344819 (closed)