Add verification dialog to confirm that native client is authorized

Each native client get a unique id which is stored accross restart. The
web client when detecting a new native client will ask the user to
confirm that the client is authorized. This prevents a malicious native
client to get the EWS access token.

In the future this could be extended to show some emojis in both the
native and web client and ask the user to confirm that they match,
currently it's only the gpgol.js + host name which is displayed.