Switch the discover and add tasks buttons.

After discussion with @denolf, we realized that most users will add tasks through "Discover tasks" rather than through the "New task" form. However in the UI, the "Discover tasks" button is hidden in the top app bar menu while the "new task" button is prominent, above the task list.

This MR switches the places of the buttons: The "Create new task" button is moved to the top app bar menu while the "Discover tasks" is now above the task list:

I also added a Cypress test for task discovery 🥳