Comments: Use `comment` instead of an empty string for the `comment_type` DB...

Comments: Use `comment` instead of an empty string for the `comment_type` DB field value in comments table.

This is the first step to bring support for custom comment types into WordPress.

Add a scheduled upgrade routine to update the type value for existing comments, in batches of 100 at a time.

Props imath, aaroncampbell, jeremyfelt, dshanske.
Fixes #49236.
Built from https://develop.svn.wordpress.org/trunk@47597


git-svn-id: https://core.svn.wordpress.org/trunk@47372 1a063a9b-81f0-0310-95a4-ce76da25c4cd