Updated service desk settings

What does this MR do?

UX tweaks to service desk settings (from #223692 (comment 413671619)):

• the error message is still about empty template if I try to pass an invalid project suffix (perhaps we can just display an error message returned in the response) - #223692 (comment 376618448)
• bellow "forward external support email address to" input field, display a note "Emails sent to <incoming_email> will be still supported" if custom_email is set (#223692 (comment 393943628))
• ideally move "project name suffix" field bellow "forward external support address..." field (#223692 (comment 377485722))

Screenshots

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.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Relates to #223692 (closed)