Replace any plain base64 with base64url

[Firehed]

Beyond making what's used where quite confusing, the standard is moving towards generally encouraging base64url (i.e. #41).

While supporting the legacy format-shifting APIs becomes a bit more tedious to base64url-shift in client code, I envision any support for it going away by 2.0 at the latest, and quite possibly before a stable release. Therefore standardizing accordingly can simplify a lot of semi-duplicated APIs.

Probably the most obvious impact is the CredentialContainer and Credential.StorageId, where the latter is url-formatted and the former is not. But there's a handful of other places with similar adjustment needed.