Add accepting to terms and privacy policy before entering an exercise

[anonym-HPI]

Right now a user could join an exercise and didn't accept that they make a connection to the server for the map in the background, which leaks their ip address. Also a trainer could change/add the URIs/URLs of images in an exercise. That way we need terms that make it clear that a trainer has to get explicit consent from each participant before he is allowed to do so.

This puts a burdon on the trainer, but else hosting the server would be problem.
I a future revision there could a popup on any client before connecting to a third party server, but that would break the game flow.
Needs to be discussed.
This fix will now only have the bare minimum for hosting.