Account deletion should not block the GUI

When confirming the account deletion, the GUI will block until the server has been fetched, the token revoked and local files purged (if the option was checked).

If the server is not joinable (user not connected to the VPN, temporary server issue, ... ) the GUI will simply be fronzen for a good amout of time. At the end, the account will be removed.

To prevent that, the whole process of removing an account should be made "async", in its own process. First, visual data should be removed and then the underlying actions should be made.