Implement the concept of membership

[odinho]

The current way we define membership is very restrictive, and not very good. Every member is expected to have a membership in some way, and they are expected to be the same. The user of the system is expected to filter the members and create custom giros for each of them. Basically membership management is manual.

It would be nice to a) connect the membership to the user in a more clear way b) allow for automating (letting the member figure out herself what type she'll probably be).

There's some issues though, how to mark that a membership exists (is "open" I guess) and how to mark it is valid? It has a start and end-date. That's how we mark it as open. But if it's paid it will be not only be open but also valid. The giros are connected to the membership, and thus you can look at the giro for say 2014 and see that it is paid, hence the membership for 2014 will be valid.

However, there's a slight disconnect between membership and giro. Because a giro can also be paid for two years (common for barne og ungdoms-organisasjonar). 100kr paid in 2013 should also count as valid for 2014.

It could be possible to mark giros with not only gjeldande_aar but maybe rather valid_from and valid_to date markers? An issue is that code will be more complex, but it would allow to have non-yearly memberships. Or in the vein of simplicity, maybe rename gjeldande_aar to valid_from_year and have an integer valid_years which will be 1 in most cases, but in the 100kr (50kr + 50kr two-year membership) it will be 2.

Also, memberships is highly different between organisations, so it needs to be replaceable. It should probably be the first actual use of behaviours.