Fixed how the Drucker Exercise works in practice

We fixed things we found by actually using it.

• You can now press "agree to the three principles" in the lobby. Pressing it puts a tick next to your name in the participant list
• You can no longer start on your own. This method is about sharing what you expect of each other, so on your own nothing is left in the report. The lobby now explains why
• The left and right arrows in the report were hard to see, so we made them clearer
• The end screen said "retrospective"; it now says Drucker Exercise

We also fixed the close button on modals shifting to the right on wide screens.