Implement backup and restore

[McPringle]

All kind of data like entities, images etc. should be part of a backup and must be restorable. Access to backup and restore has to be secured (admins only).

Export all entities as JSON files. One JSON file per entity type (event, sponsor, SIG etc). Export all images as files in folders per entity type. Compress all the backup data using ZIP compression and use the date and time as a part of the filename, e.g. komunumo-backup-20170915180000.zip.

The RESTful JSON API:

• GET /api/backup: Create a backup and send it to the client (secured, admin only)
• POST /api/backup: Restore a backup (secured, admin only)