WIP: validation message generalizations

@talbs I have these code changes (which are not close to being ready for code review!) on my sandbox. Here is a test course you can use (staff@example.com):

http://studio.cahrens.m.sandbox.edx.org/course/brian/brian/brian

I wrote up several questions and thoughts in https://openedx.atlassian.net/browse/TNL-704

There are also some comments directly in component-validation-messages.underscore
And if you add `__add__`, then you get an easier interface for concatenating the messages from the superclass. Really, I almost want `ValidationMessages` to just extend/mimic lists, but that may be more work than it makes sense to add right now.