use list_transports

[r10s]

• as a preparation, get rid of all oauth2 things; they are unused and clutter code remove deprecated oauth code #3977
• remove legacy "companion app" code remove deprecated companion app #3981
• to get prepared for mutli-transport, use list_transport instead of CONFIG_ in RegistrationActivity
• rename RegistrationActivity to EditTransportActivity
• add a ListTransportActivity similar to the proxy view that allows removing transport and selecting default transport
• adding transport, for now, should be done by scanning an DCACCOUNT:-QR code from the normal scanner (instead of adding a new profile)
• changing the address should not be possible in the settings of the transport. If you want to change the address, then create a new transport and remove old one.

cc @link2xt does that make sense for the current state of multi-transport? otherwise, feel free to edit this issue directly :)