Import the whole datetime module.
Removing members doesn't work, remove it until implemented correctly.
Simplify the way we call active_member.
Save the start and end dates of membership in the database.

We now save a membership's start and end date instead of whether a user is
a member. This allows us to distinguish between former members and new
member applications. User.active_member is still accessible but computed
based on the saved dates. Saving the dates will also allow us to compute
who our members were at a previous point in time, which is a legal
requirement.
Improve error message when membership is required.