Support expiration date for access code

[damianhxy]

It would be good to support automatically disabling access codes after a certain date. That is, add a new date field to course config along the lines of access_code_until.

Some thoughts

• Could set it to ~2 weeks after start date, to roughly match with add deadline.
• When access code is enabled but expired, display a different message on the course page. Something like "access code automatically disabled"

Also, consider preserving the access code when exporting a course, i.e. add access_code to GENERAL_SERIALIZABLE in course.rb. However, it might make more sense to regenerate the access code (if one was set). In which case custom logic would be needed to persist the fact that an access code existed.