Fernet tokens were previously percent encoded. This can cause issues with
clients doing their own encoding or not. By removing the padding and then
re-establishing it when we validate the token, we don't present that problem to
clients. This also shortens the length of a Fernet token.
Change-Id: I674bad86ccc9027ac3b365c10b3b142fc9d73c17
Related-Bug: 1433372
Closes-Bug: 1491926