Enable deleting an existing user and generating custom db ids when
inserting new users

- Add delete-user function to mongo.clj file and modify users.clj file
to enable deleting
- Add next-seq and insert-with-id functions in mongo.clj file for
generating custom db ids and inserting them to db
- Modify users.clj file to enable reloading a user list
- Move verify-register-form function from mongo.clj to register.clj file