Add a feature to combine existing groups

[tomca32]

We got a feature request from Ryan to merge two groups into a single group. I think this should be possible but we need to think about everything that needs to be changed.

I can see that the biggest problem would be if groups had lessons on the same day. These should probably be merged into the same lesson, and there will be some grades updating there to make sure grades belong to the correct lesson.

Also there should be some sort of UI there showing how the groups are merged...like if "A" is merged into "B" or the other way around, with the option to flip the merge direction.