Fix #66 invoke commands for media file backup/restore

[jifox]

Added the commands:

• invoke backup-media: to backup all media files in media.tgz

    help={
        "media-dir": "Media directory to backup (default: `/opt/nautobot/media`)",
        "output-file": "Output file, overwrite if exists (default: `media.tgz`)",
     }

• invoke import-media: to import the media.tgz file and restore to it's location (media-dir at backup time)

  help={
      "input-file": "Tar file containing media files from backup. This can be generated using `invoke backup-media` (default: `media.tgz`).",
  }

[jifox]

There seems to be no interest to merge