Fix #11625 - Can't delete related issues if it has more than 3 links

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/11625

The "add related issues" form was keying pending issues by their index in the form. This confused Vue, because indices don't map 1-1 to pending issues if an issue is removed.

Visual explanation of what's up:

This MR also disallows duplicate pending issues to be added to the form if they are already in the form.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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