'Switch stage' should allow to move backward stages only

when using 'switch stage' to move forward the stages several problems pop up:

• chip config files do not get generated
• some users may be tempted to move forward to a much later stage and then click 'submit to ITkPD' on the module page several times to sign off the previous stages. This leads to stage coherency issues which are not well understood

therefore I suggest to limit the 'switch stage' functionality to move backward through the stages only.

In case a user wants to move to later stages he should sign off the stages in between by selecting 'no test to upload' even though he plans to go back to it later.