Fixes and improvements

Course statistics

• Fixed rare instances of pass rate going over 100% on the Attempts table
• "Course" and "Substitutions" texts are now hidden if there are no substitutions for the course

Class statistics

• Removed Students tab from Courses of Class
• Removed Passed after retry and Failed multiple times columns from Pass/Fail tab

Internal

• Course statistics
  • More testing
• Updater
  • More error handling