Add a `memberTo` field

[foodelevator]

Closes #12

[foodelevator]

The membership list we get from THS reportedly has an expiry date for each user. I guess everyone most of the time should have the same though as you pointed out. But if we would set everyone not on the list membership to false we it would be harder to manage people we think are members but perhaps THS doesn't. I guess e.g. those in Ordförandeordern would be affected here

[foodelevator]

I think I agree. My thought was that it would be easier to check if someone is a chapter member if permanent members get an and date far into the future, but it still pretty easy:tm: to check a bool and a date ^^