feat: add Infinite scroll to "Add Projects" modal in the operations dashboard

What does this MR do?

Adds Infinite scroll to "Add Projects" modal in the operations dashboard

Current behavior: When a user enters a query that results in more than 20 projects, they will only see the first 20 projects, since the search only brings back the first page of results.

Screenshots

Oct-10-2019_18-53-55

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

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

Part of #30518 (closed) Must be merged after gitlab-ui!736 (diffs) is merged