Add user delete database migrations

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.

migrations/009.user-deleted.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE users DROP COLUMN IF EXISTS is_deleted;
+ALTER TABLE users DROP COLUMN IF EXISTS deleted_on;
+
+DROP INDEX IF EXISTS users_username_idx;
+ALTER TABLE users ADD CONSTRAINT users_username_key UNIQUE (username);

migrations/009.user-deleted.up.sql

@@ -0,0 +1,5 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS is_deleted boolean NOT NULL DEFAULT(false);
+ALTER TABLE users ADD COLUMN IF NOT EXISTS deleted_on timestamp NULL;
+
+ALTER TABLE users DROP CONSTRAINT IF EXISTS users_username_key;
+CREATE UNIQUE INDEX users_username_idx ON users (username) WHERE NOT is_deleted;