Make PhrictionContent "description" non-nullable

Summary:
Depends on D19095. Ref T6203. Ref T13077. This column is nullable in an inconsistent way. Make it non-nullable.

Also clean up one more content query on the history view.

Test Plan: Ran migration, then created and edited documents without providing a descriptino or hitting NULL exceptions.

Maniphest Tasks: T13077, T6203

Differential Revision: https://secure.phabricator.com/D19096