Pager: scroll to top after page has been loaded
Wrote class for single tasks

This resulted in the following changes:
  * Most problem-relevant code is now in one place (the class Task)
  * Tasklists as on the main page and the task page now share the code
    to display the task. Therefore, they also share their design now.
  * Tasks on the main page are no longer wrapped in <a> tags, which was
    against the standard. Instead, they now have a button. We also
    changed how information about the problem was displayed in the last
    line.
  * We moved the publish form out of the problem box.