Resolve "Opening Project with invite but without accepting leads to 404 error page"

What does this MR do?

When an external user get invited to a project/group and signs_up we automatically accept all his pending invitations and redirect the user to the dashboard_page. If the user signs_up using a different email we redirect the user to the accept_invitation page.

Step to test this feature

• Create a private project/group
• Go to the project/group members page
• Invite an external user
• Sign up as the external user using the same email you've been invited to
• You should see the dashboard page with the project you've been invited to join

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42531 (closed)