organizations: Allow users to create organizations as shared users

These organizations allow a user to decide to become an organization they are a member of, as well as manage users organizations they own.

Schema changes:

```sql
alter table session add user_as_id INTEGER;
update session set user_as_id=user_id;
alter table user add type VARCHAR(32);
update user set type="user";
```

`create-all.py` should take care of the rest.