#42 Add "teaching game" support

[Alf Mikula]

Add "teaching game" support to game offers. This feature is for players of all levels to be able to find opponents much stronger than themselves who are willing to teach.

Scheduled games must be explicitly marked as teaching games in these cases, so the accepting user should have the option of checking a box to accept the teaching game instead of a competitive game. No results should be reported for teaching games.