This commit removes some downgrading options that lead to game crashes like going from a Soyuz L.L. to an LM test or going from joint mission to single launches. I made a few judgements calls concerning what is allowed and what is not:

1. Going from LOR to EOR is not allowed. The rationale is that this is not really a downgrade, but a new mission with additional steps not present in the original one.
2. Any downgrade that might require new hardware is not allowed. This rules out to go from Joint LM tests to Joint Docking Tests, as this might require an additional docking module depending on the capsule flown. If you don't go back to reassemble your hardware, this might lead to problems again.

Fixes #713 and fixes #726.