Keep some files automatically synchronized, without having to do it manually

### Is your feature request related to a problem? Please describe.
I'll take a minor example : I'm keeping a "memes" file on my Nextcloud, so I can save memes and reuse them quickly on social networks. But on my Nextcloud app, I can't seem to find a way to keep files permanently synchronized, I have to do it by hand. Which greatly impedes my meme-game on Twitter.

More seriously, it would really help to have my files permanently sync and download on my smartphone.

### Describe the solution you'd like
A feature to mark files to be automatically synchronized periodically, so the list updates and they download on their own.