Fixes and Tweaks to Lesson Plan

Fixed an issue with the display of items on the main lesson plan - the helper method used to determine what items should belong in a particular milestone uses a closed interval, so an entry starting on the exact date/time that a milestone ended at would get displayed in both that milestone and the one it actually belongs to. Added a another filter step in the controller to prevent this.

Fixed the entries in the overview not being sorted, and also found and fixed an issue preventing virtual milestones from even appearing in the overview - moved the overview method from the milestone controller to the entries one, since it needs much of the same logic used there. It also seems to make more sense there, since the handling of the main lesson plan is already in the entries controller anyway.

In addition, I added delete buttons for both milestones and entries to the overview. Changed the redirects in the delete methods in both of these controllers to go ":back", so they don't always dump you back on the index.