Frontend: Use custom user name for service desk emails

What does this MR do?

Adds frontend support for custom user name in service desk emails. Default value (GitLab Support Bot) is shown via placeholder on UI in case user has not provided any custom name. Backend for this feature was done in !22090 (merged).

Screenshots

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

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

#7529 (closed)