Migrate members

This issue keeps track of everything we need to pay attention to when migrating the members.

• Memberships have been expanded. Use the current membership types + joining years as starting points, and fix individual members as appropriate.
• 'registration year' is now 'starting_year'; this is the year someone started studying, but not necessarily the year they joined Thalia. However, for lack of information, during migration we only have one year value to work with..
• We will need to re-hash the passwords. We can probably do this without resetting everyone's passwords.
• Add migration instructions to migration.md