admin: Compo entry mass exporting and manifest generating

[katajakasa]

prerequisite: https://github.com/Instanssi/Instanssi.org/issues/289

There should be a feature for exporting all entries in a compo and an additional manifest file.

• Make an async entrypoint that allows streaming out zip data. Read files and write them to zipper (or tar.gz) as they get read. Perhaps put them into a directory by compo name too ?
• Manifest should be specific for the entry type (possibility to select one?). Manifest type could be autohinted to select box by checking file extensions.
  • m3u8 for music, track numbers assigned as order on the screen presentation
  • txt (simply list of entries, sorted in the same order as on screen presentation
  • Possibility to write no manifest
• Rename files so that they have the list-index as the first part of the name, so that windows/gnome/etc. sort algorithm can sort them out (e.g. --..

[zertap]

Should also include the user-fillable text fields. People might put instructions to the description field for example.