Make fetching of membership types lazy
Make fetching of membership types lazy

Otherwise migrations break since the table does not exist when we
try to load the data, as we are just about to create the tables...

ojousima/asylum,hacklab-fi/asylum,HelsinkiHacklab/asylum,ojousima/asylum,rambo/asylum,jautero/asylum,hacklab-fi/asylum,HelsinkiHacklab/asylum,jautero/asylum,hacklab-fi/asylum,HelsinkiHacklab/asylum,HelsinkiHacklab/asylum,ojousima/asylum,jautero/asylum,ojousima/asylum,rambo/asylum,jautero/asylum,hacklab-fi/asylum,rambo/asylum,rambo/asylum