Allows labels to be added to assigned AssignmentPlans

This should address the need for manual database manipulation in the event that Assignments are issued from an AssignmentPlan without the labels needed to properly customize the associated LearningConditions (which is happening fairly often in the case of tests).

Before an AssignmentPlan results in Assignments being issued, the process for updating labels is the same as before (they can be added/removed in the AssignmentPlan edit dialog).

Once the AssignmentPlan has issued Assignments, though, labels can only be added via a new field in the edit dialog (existing labels are read-only):

![Screen Shot 2013-03-22 at 7 00 38 PM](https://f.cloud.github.com/assets/1851667/294177/5753cce0-93da-11e2-8581-a8c6c84044f9.png)

These new labels will be immediately applied to any existing AssignmentExercises associated with the AssignmentPlan, allowing updates LearningConditions to target them.

As an implementation detail, AssignmentPlan now `acts_as_taggable`, and there are database migrations to do the conversion.  @jpslav