E2409. Refactor sign_up_sheet_controller.rb

[Harshil47]

1.The previous version of the project fixed the naming discrepancy issue. We have validated it and found no other changes.
2.Refactored delete_signup and delete_signup_as_instructor - to create a centralized the shared functionality, delete_signup_common, a new method which lessens redundancy and enhances maintainability.
3.Refactoring the Update method to make it more concise and readable. It uses the Rails 'update_attributes' method to update multiple attributes of the @topic object in a single call. It also sets a success flash message when the topic is updated successfully, which was missing in the original code.
4.Refactoring the signup_as_instructor_action method - We have reduced the function length which was raised in the Code Climate issue. Also added multiple helper function to reduce the if-else ladder complexity. We added a new user_registered_for_assignment, process_signup_as_instructor_request helper functions as well as log_message function to make sure DRY principle is followed while logging messages.
5. Refactoring the save_topic_deadlines method Part-1 - As part 1 of refactoring save_topic_deadlines we will be reducing the number of lines of code for this function as it exceeds the limit which was raised in code climate. For this we gave create separate create and update methods of creating and saving TopicDueDate object. Additionally instance_variable_get('@assignment_' + deadline_type + 'due_dates')[i - 1] and instance_variable_get('@topic' + deadline_type + '_due_date') were used multiple number of times