Quiet on mobile with auto-download

[kingalg]

Version: 3.0.0, 4.0.0
System: iOS, android

Description: We have a working auto-download on mobiles as a side effect of other changes. Files below 20MB are downloaded automatically and accessible in a Quiet folder.

Issues:

1. Downloaded files can't be opened through the app.
2. Bigger files that are not auto-downloaded are visible in the app UI as downloaded as well (but only if they were uploaded from a desktop; they don't have a false "Downloaded" status if they are uploaded from mobile).

There are two possible solutions:

1. Disable auto-download and fix UI telling users on mobile that files are downloaded.
2. Treat working auto-download of smaller files as a feature and add appropriate UI.

I vote for option 2. What do you think, @holmesworcester? There is a matter of memory on mobiles, so some kind of decision about whether users want to sync everything/some/none of the files would probably be a good idea to add in the future. I like how it's organized on WhatsApp.

I assigned it a type "bug" because currently, it shows users the files' untrue state, but if we decide to work on this as per point 2, it'll be more of a feature.