Potentially misleading error message when role latest expiry has passed

[jc-harrison]

When creating a token in FlowAuth, the token expiry is set to the latest expiry permitted by all selected roles. If the "latest expiry" for one or more of the selected roles is in the past, this will result in the token having already expired, which will result in an error message "Token for <username> expired".

This could be confusing, particularly for users less familiar with the terminology of FlowAuth permissions concepts (users/roles/tokens) - it may give the impression that the user's FlowAuth account has expired and they are blocked from creating tokens, rather than that the token they attempted to create has expired.

Some re-wording of the error message may help (e.g. "Error: New token has already expired!"), but it may be preferable to prevent the user from getting to this situation at all - e.g. by greying out roles whose latest expiry has passed.

It would also be useful to display the token expiry before clicking "save", so users can see how long the token they are about to create will be valid. Although there are some tricky aspects to this as the expiry depends on which roles are selected (as mentioned in #5719).