Password protected task user interface

Implement the frontend functionality for backend-core#60 (closed).

• On the student UI, the password could either be requested on the file upload form (requires refactoring) or on a separate page after clicking the Upload button.
• On the instructor UI, the password should be given by the instructor creating the task. The password should be modifiable and an existing task should be made password protected after its creation. (Vica-versa, the the protection should removeable.)

Note: there is a detailed description about the current implementation on the page of the related merge request (!40 (merged))