Convert to urlsafe base64 audit ids

Convert audit ids over to being urlsafe base64 strings. This also
addresses an issue with Python3 bytes object not having a .encode
method.

Change-Id: I9e9459b87ebb84d3bc3e36ef10a4b8364abc0c5f
bp: non-persistent-tokens
Merge "Convert to urlsafe base64 audit ids"