Add multi account support and scheme URL for switching accounts

[mpivchev]

We should:

1. Add multi account support. Currently only one account can be logged in.
2. Add a scheme url for switching accounts, but only if that account is logged in, otherwise do nothing. In the Files app we use this url scheme, so ideally it should be the same:

nextcloud://open-and-switch-account?user=someUser&url=https://cloud.nextcloud.com

It's a feature request connected to: nextcloud/ios#2460

[i2h3]

This is superseded by the planned issue #88.