Add custom email templates for service desk

What does this MR do?

It adds the possibility to customize templates for service desk emails.

In this first iteration, a user can add files to the project repo and they will be used instead of default templates. The files have to be stored in .gitlab/service_desk_templates dir and be names as thank_you.md or new_note.md. Basic placeholders ISSUE_ID, ISSUE_PATH, and NOTE_TEXT can be used.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Issue #2460 (closed)