Add WebDAV presets to ChooseCloud screen

[tobihagemann]

With the addition of "Other File Provider", it's possible to assume that "all other" cloud storage services can be added there. However, in reality, most cloud storage apps don't support opening folders via file provider and are greyed out in the DocumentPicker.

Many users don't necessarily know that their cloud storage support WebDAV. That's why we could add more cloud storage services to the ChooseCloud screen without actually having to implement each proprietary connection. Here is a list of well-known WebDAV addresses that we could use as presets.