feature_request(backup): synchronization

### 1. Summary

It would be nice, if would be possible to synchronize notes and annotations with any free remote cloud (examples: Box, OneDrive, Dropbox) or use technologies as [**WebDAV**](http://www.webdav.org/).

### 2. Argumentation

It's obvious to me why synchronization is needed.

1. My account can be hacked and all annotations deleted.
1. Your strategy may change. The technical support of the web service, which I used in the past, promised that it will never close, but it closes and I lost about 500 annotations.

[**More details here**](https://blog.storagecraft.com/top-five-reasons-to-back-up-your-data/).

### 3. Not helped

I don't find, how I can synchronize data, in my Hypothes.is settings.

In Google SERP I find only [**tool in John Udell personal site**](https://jonudell.info/h/facet/). But it is backup, not synchronization. Hypothes.is user need manually make data backup, the process is not automated. Manual synchronization may take many user time.

### 4. Examples in another apps

[**Joplin**](https://opensource.com/article/17/12/joplin-open-source-evernote-alternative) allows synchronizing data with local file system, OneDrive, Nextcloud, WebDAV and Dropbox.


Thanks.