Liana Connect: "Add/Join existing wallet" flow

[nondiremanuel]

This issue splits from #970

The entire flow of the installer will be reviewed following the schema below:

This issue is to manage the "Add/Join existing wallet" flow, which will condensate both the "participate" and the "restore" flows. This flow will allow the user to either participate to a wallet or restore one both by using their own backend and by connecting to Liana Connect (remote "Liana Lite" backend).