Set a default value for UNREAD

This isn't exposed anywhere, but I'm trying to add some "NOT NULL"
constraints to the table and this one is necessary.
Add many "NOT NULL" constraints to the database

So it turns out that pretty much nothing can be NULL in my SQL
database.  This adds an additional constraint to pretty much every
column that attempts to enforce this rule.

Note that while I'm running this now it may be buggy.