[BugFix] Fix Users not deletable because of foreign key reference (#300)

Fixes #297

db/migrate/20191126185833_update_user_invite_foreign_key.rb

@@ -0,0 +1,6 @@
+class UpdateUserInviteForeignKey < ActiveRecord::Migration[6.0]
+  def change
+    remove_foreign_key :users, column: :inviter_id
+    add_foreign_key :users, :users, column: :inviter_id, on_delete: :nullify
+  end
+end