Improve message when user declines a grant

[RabebOthmani]

Within the WM extension, when a user sets up a grant or modify it, they are directed to a screen where they need to confirm or decline the grant.
In the case when they decline, they get a message saying "Your request was successfully rejected" on the /welcome screen.
Having successfully and rejected side by side feels odd, I wonder if we can phrase this better. Help @melissahenderson :)

See interledger/web-monetization-extension#577 for more context.