Use join for badges on student dashboard.

Use an ActiveRecord join to select the badges for the student dashboard. This makes a single trip to to the database to obtain all of the records necessary to display the badges table of the student dashboard. No longer using the momoization within badges, so it may not be necessary if it is not used elsewhere.

Note that the report is different. View the report with the current code and compare it with these changes. You'll see that the "earned at" time is present for more than just the first badge.