please provide a button to renew other's memberships

[Mattia Rizzolo]

Currently when a team administrator needs to renew somebody's membership because they are about to expire, he needs to go through these steps:

1. open the ~team/+members page
2. identify the expiring members
3. hit the modify button (~team/+member/username)
4. modify the expire date
5. save

I believe steps 3-4-5 could and should be optimized, therefore I'm proposing the following improvements:

* add a "renew" button in the ~team/+member/username page, that automatically moves the expiring date behind by the time set in the team configuration page, and saves.
* highlight the expiring members (<= 7 days from the expiry date?) in the ~team/+members page, and also provide said renew button.