Improve `validation.h`

[RagnarGrootKoerkamp]

• should write to judgemessage.txt and teammessage.txt, instead of stderr. Reserve stderr for crashes and possibly judge_errors.

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Less obviously good changes:

• Validator object could be global -> I actually think a local variable is nicer.
• streams can be global and opened by Init function -> hidden global initialization is somewhat confusing actually. You'd still need to know the names of the variables.
• possibly move main to the header and make the cc only implement a single function -> same as above.