[2.x] inject UriSigner directly in the VerifyEmailHelper

[jrushlow]

• We can inject the UriSigner directly into VerifyEmailHelper instead of using a factory to pick the correct service.
• We can remove xor deprecate the UriSignerFactory in a separate PR for 2.0 depending on what we do with #165
• replace @final docBlock tag with final class keyword in UriSignerFactory