Update surveys_controller.rb

Two method have updated
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1) reminder : send reminder to individual user who is not completed
2) reminder_all: send reminder to all the students who are not completed yet

Usage:
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Both functions are useful for lecturer and students as students will be notified to complete their survey. Lecturer does not required to remind students during class. Easy electronic method and prompt notification.

Note: Can combine both function into one method in future -- Need to fix but not urgent