(hopefully) Final fixes and prettifys

[ADimeo]

As discussed:

• Remove "results" button on unstarted polls for students
• switch message "you stopped the poll" and "you started the poll"
• Students can see poll results after the lecture ends, but the button is missing for lecturers. Please add the button for lecturers.
• If feasible: Remove checking of changes to polls and to list of students if lecture is already archived
• If feasible: Only send requests for list of students/changes in polls if user is currently in that tab