Apart from adding the delete-related columns, this migration replaces
the existing unique constraint on the username with a unique index
which has a WHERE clause, so that deleted usernames need not be unique.
This is to avoid having to implement a special function for generating
unique IDs just to keep the deleted username unique and under 30 chars.
The deleted username can still use e.g. the initial part of the user_id
but the uniqueness is not required.
