not all articles should show up in Recent Additions and Recent Revisions boxes

[holtzermann17]

Anything that isn't Reference should only show up in the box on the lower right. This provides a line of defense for "community standards". People can argue about classification of articles if they want, but some of the things that have been posted to the encyclopedia aren't really ready for the prime time yet.

This starts to deal with some issues that would be tackled on a larger and more thorough scale in #49.