refactor: rework media management

This refactoring includes some fixes, new features,
and updates to the API:

* the new /media and /media-sources APIs are used
* users can now cancel the addition of new media sources
* all errors should now be catched and displayed, no matter when
  in the handling process they happen
* users can show the import logs for uploads
* uploads happen in the background so users can move to
  other pages without aborting the upload

refs #347
refs #328
refs #323
refs #342
refs #273