Change assignee dropdown invite to utilize invite modal

What does this MR do?

• For maintainers and owners, changes the Invite Members link from sending you to the project members page -> opening the invite members modal instead.

Test locally with GDK

• Go to any issue or merge request where you are an owner/maintainer
• Open assignee dropdown and click Invite Members.
• Ensure you see invite modal launching.

Screenshots (strongly suggested)

Before

Screen_Recording_2021-03-24_at_5.05.48_PM

After

Screen_Recording_2021-03-24_at_4.40.59_PM

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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 #322879