Adds diagnostic info for assignment preview page

This PR adds PresentationCondition- and FeedbackCondition-related diagnostic information to the Assignment Preview page.

The following table is displayed per AssignmentPlan and Cohort:
![screen shot 2013-09-10 at 8 58 53 pm](https://f.cloud.github.com/assets/1851667/1120206/91cf22dc-1a96-11e3-8d81-e37b0a508d49.png)

The `Ex Labels`, `PC Id`, `PC Regex`, `FC Id`, and `FC Regex` columns are populated with `N/A` if the current user is not an Admin or Researcher or if the klass is not part of a controlled experiment.  The table is too wide to display effectively on narrow screens, so some horizontal scrolling will be needed for users working on laptops, etc.

On the LearningConditions page, the IDs for the LC's PCs/FCs are listed along with their other properties, making it easy to correlate information between the LearningConditions page and the Assignment Preview page.

The LC's `get_{presentation,feedback}_conditions` methods have been moved from `protected` to `private` to allow them to be called from other classes.  This will be used extensively in upcoming PRs.  Also, the `applies_to?` methods on PresentationCondition and BasicFeedbackCondition have been tweaked to accept either a StudentExercise or and AssignmentExercise.  This will greatly simplify code in other parts of the system (like reports).

There are also some minor tweaks to the page headings for other Klass-related pages, mostly to make it easier to find the appropriate tab in a browser.