Add error when previously-assigned exercise is reserved for test

This PR should close issue #111.

When an Educator tries update a TopicExercise by reserving it for tests, an error alert will appear if that TopicExercise already has an associated AssignmentExercise (meaning it was previously assigned).  This validation is only takes effect if :reserved_for_test is changed from false to true, and is not run upon creation of a new TopicExercise.

In addition to adding the validation, this PR expands the size of the TopicExercise new and edit dialogs to allow room for an alert without the need to scroll.