User management - account should expire

[jschnurr]

When an admin sets a role for a user, it should prompt for an expiry date, defaulting to one year. After the expiry, the role is revoked. This will help with seasonal turnover of students and avoid piling up years of old accounts.

[LauchlinMorrison]

Do we default it to 1 year from the date or do we have it roll over at the start of our next season? (Either around registration time or kick-off time?)