Onchain notifications should link to etherscan

[czepluch]

Notifications in the webui that are related to onchain events should have a link to etherscan where users can follow the process of the onchain action.

[manuelwedler]

This needs a change in the API, as the API calls at the moment only return after an on-chain transaction was mined. I created an issue on the python client repo: raiden-network/raiden#7125