add models for attaching notes to servers and applications

Figured it would be nice to be able to write short notes on servers and applications. So a very simple model with user/timestamp/text. A logged in user will be able to just quickly add a note and the server/application page will then show all the notes. I'm inclined towards this timestamped note approach instead of a single wiki-ish field on each model because I think it's handy to have the history with dates and user attribution right there next to every note. This will also give us a way to log simple events like "LITO moved this server to a different blade". Manually for now and maybe eventually via automated scripts (automatically log reboots, etc).

This commit is just the model additions and a migration to get them deployed to production before any actual feature development happens.