Add Loaders page

Merge Request Checklist

• Link to an issue 2071 - Add new Loaders page to UI
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add a CHANGELOG.md entry in the Unreleased section

Once your merge request is complete, please assign it a Meltano maintainer for review cycle. Once the review cycle finished, the reviewer shall approve the merge request so it can be merged by any Meltano maintainer.

Closes #2071 (closed)