Add expiration to FacebookConnectorOAuthState

[ddossot]

It's currently impossible to know when the token will expire: Facebook provides this information alongside the access token so it would be nice to store it in the persisted state.

[3miliano]

What do you need it for?

[ddossot]

For example to be able to display "You have successfully authorized the connector, the token will be valid ...". This allows the end user to confirm if he got a short lived or a long lived token.