Conditionally import loads the projects dropdown when opened.

What does this MR do?

Loads the projects_dropdown only when the dropdown is opened.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3840

Are there points in the code the reviewer needs to double check?

Check that the project dropdown events are being dispatched appropriately.

Why was this MR needed?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/38869.

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?