Replace bootstrap modal in issuable_by_email HAML template

What does this MR do?

This MR replaces the Bootstrap/HAML implementation of https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/projects/_issuable_by_email.html.haml with a Vue component and mounts the component on the project's issue list and the project's MR list.

Screenshots (strongly suggested)

Issues	Merge Request
issuable_by_email_issue	issuable_by_email_merge_request

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

Related to #254256 (closed)