IterateAll Functional Improvements

A number of outstanding tasks remain for the IterateAll functionality:

- When uploading files during a time at which the preview processor is disabled or not fully working, they will continue to have `status: pending`. There files will not actually be re-processed by the current `/api/content/reprocessPreviews` endpoint
- The `/api/content/reprocessPreviews` endpoint should be moved out to the `oae-preview-processor` module
- The Global Admin UI should be given a new option in the left hand navigation, that then offers buttons for Re-indexing everything and Re-processing all previews.