.#23 Better guidance related to task completion and accessing a compl…

[Kenneth-Funes-TU-Delft]

…eted task

• Better guidance to the user when the task is completed. Not showing the completion code, but redirecting to prolific on submit
• Killed the existing prolific academic pop up which was being eventually called with the props.onSubmit logic, and replaced by local logic with better user experience
• Created a TaskAlreadyCompleted component that blocks access to the task (and data therein) when the results have been submitted to covfee
• Minor text improvements.