Default model repr should omit id when other unique columns are present

[EliAndrewC]

Moved here from the original (now-private) issue tracker.

SQLAlchemy models in Sideboard get a default repr which by default displays all unique columns. So when going through the Sideboard tutorial, you'll see something that looks like

[<Website id='9d4eb9f2-3b92-4ddf-9b22-999b2063d98b' url=u'http://hastheworldendedyet.com'>, <Website id='cc06e57b-eb3d-484c-908c-bb4f97b81306' url=u'http://hasthelargehadroncolliderdestroyedtheworldyet.com'>]

This would be far more legible if it just read

[<Website url=u'http://hastheworldendedyet.com'>, <Website url=u'http://hasthelargehadroncolliderdestroyedtheworldyet.com'>]

My opinion is that we probably don't want the id to be present in the default repr when there are already other unique columns.