Removed 'Dismiss this assignment' button

This might be a contentious MR, so this is also intended to serve as a place for discussion.

It has happened a few times that someone accidentally dismissed the whole assignment instead of a single game. This can be quite annoying, since there is no possibility to revert it. Considering that dismissing the whole assignment is seldomly very useful I think it is best to just remove the corresponding button.

(This MR leaves in the code and the action page for dismissing the assignment, so it is still possible if one knows the correct URL. This could be removed as well; on that I have no strong opinion.)