Add invite codes / counts to users table

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/collab/migrations/20220518151305_add_invites_to_users.sql

@@ -0,0 +1,7 @@
+ALTER TABLE users
+ADD invite_code VARCHAR(64),
+ADD invite_count INTEGER NOT NULL DEFAULT 0,
+ADD inviter_id INTEGER REFERENCES users (id),
+ADD created_at TIMESTAMP NOT NULL DEFAULT NOW();
+
+CREATE UNIQUE INDEX "index_invite_code_users" ON "users" ("invite_code");