Sync/Export comments as separate JSON file

**Is your feature request related to a problem? Please describe.**
I need to keep a log of comments - both public and private - for each note, for auditability reasons.

**Describe the solution you'd like**
An export feature - or, even better, autosync with a GitHub repository associated to the note - so that comment threads could be saved (and verison controlled) as a separate JSON file..

**Describe alternatives you've considered**
I'm a new user, and couldn't find any feature like the one describe in the current Hack.io web interface. An alternative would be to use another solution - like [CodeStream](https://www.codestream.com), which is less friendly for non-techie users.

**Additional context**
I work at a government department where we are searching for a tool that 1) creates documents in HTML format by default, without (much) extra configuration; 2) has a friendly interface (wysiwyg) for non-techie users; 3) allows collaboration both privately (e.g. for preparatory documents, that should be publicized later) and publicly (even without registration - e.g., for public consultations); and 4) allows all changes and decisions to be audited (version control). Currently, Hack.io seems to fulfill almost all these requirement - the only exception being auditability of comments.

L’issue non indica alcun file, test o punto di ingresso. Chiarisci innanzitutto se l’obiettivo è un’esportazione una tantum o la sincronizzazione automatica con GitHub, quindi definisci la struttura JSON e come vengono gestiti i thread di commenti pubblici e privati. Il lavoro è completato quando il workflow scelto conserva la cronologia dei commenti per l’audit e il controllo versione.

Scritto dal modello di indicizzazione a partire dal testo della issue.