Make unpublish date optional

Problem/Motivation

The unpublish date has a max of Dec 31, 2050. The field is required. I think it'd be a good idea to either remove the max date, or make it optional.

Steps to reproduce

Create a notification message. It can't be made without an unpublish date, and the date can't be made something like the year 9999.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes