Syncs (downloads) continue over mobile data per default without a warning or ability to configure

### Actual behaviour
The app synchronizes over mobile data per default WITHOUT a warning or asking about it even once - that's bad since traffic-limited plans (with either significantly degraded functionality or highly increased cost after exceeding the overall traffic volume) are still the norm for many European countries, and I assume also most of the US.

### Expected behaviour
Sync over wifi only, or make a big(!) popup at the start that warns me about this default setting of syncing over data and offer an obvious button to turn that off.

### Steps to reproduce
1. Install & open nextcloud app
2. Login to a cloud
3. Choose an existing folder on the server with existing content on the server (a non-trivial amount of files), which isn't present / synchronized to the phone storage yet. Click the "Sync" button from the folder menu on the right. The app should now start downloading things to the phone storage
4. Get out of wifi range with mobile data enabled, and observe that synchronization / downloading will continue

### Environment data
Android version: 7.1.2

Device model: GT-I9195 / serranoltexx / Samsung Galaxy S4 Mini LTE (International version)

Stock or customized system: LineageOS 14.1

Nextcloud app version: 3.0.3 / f-droid

Nextcloud server version: don't know

### Logs
#### Web server error log
issue seems to be server independent, currently not having any access to the server I tested with

#### Nextcloud log (data/nextcloud.log)
issue seems to be server independent, currently not having any access to the server I tested with