Remove code-duplication around building AuthActions

The AuthActions only differ on what criteria they have for forcing the user to sign-in, so can remove a bit of repetition.
> The logic here will pass through an expired identity to the block.

No it won't! See this line in the AuthActionBuilder:

https://github.com/guardian/play-googleauth/blob/0e63aea/module/src/main/scala/com/gu/googleauth/actions.scala#L71

...specifically the `identityOpt.filter(_.isValid)` bit - I agree that calling code should never get a user-identity that is in any way invalid, for instance expired.