Feature request: Download your data

i'd like to see a 'download your data'-function in hackmd similar to google takeout.

reasons:
- it's a nice thing to have

legal: offering hackmd-services to europeans comes with legal responsibilities (general data protection regulation GDPR)
- offering users the possibility of backups and admins an export function ( right of access, Art. 15 GDPR)
- possible data portability by uploading a backup to another hackmd-instance ( data portability, Art 20 GDPR)