Display posts and users inline in insights notification email

Document Grunt handling email CSS inlining.
Hey, posts work pretty well. Also, I improved the fake post generator
for testing.
We now have users in emails, a fix for some posts in emails not
appearing, and the ability to generate many test users.
Fixed a handful of broken tests and wrote new ones for posts and users
in emails.
Format data
Throw an exception for unserialize() errors
Debug failing test on 5.2