Copy forward email address in issue sidebar

What does this MR do?

Backend MR: !49050 (merged)

This adds the ability to copy the forwarding email address to the issue when you're logged in.

Related to #18816 (closed)

Repro

• in config/gitlab.yml go to incoming_email and toggle enabled: true
• set address: gitlab-incoming+%{key}@your_last_name.com
• Restart GDK
• Go to an issue (logged in), scroll down to the sidebar
• Click the copy email address button

Issues

• The clipboard_button.vue component is not rendering the tooltip when you specify the position. We will need a follow up ticket for this. #296671 (closed)

Screenshots (strongly suggested)

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