A notification record is associated with a user, and shows up in their
notification list. A notification indicates that something has happened that
the user should know about, like if a user's discussion was renamed by
someone else.
Each notification record has a *type*. The type determines how the record
looks in the notifications list, and what *subject* is associated with it.
For example, the 'discussionRenamed' notification type represents that
someone renamed a user's discussion. Its subject is a discussion, of which
the ID is stored in the subject_id column.