Add a do_not_pair_with option

[faboshka]

Currently, anyone could be chosen to be the secret Santa of anyone else from the participants.

While everyone in my friend group are fine and happy with this - there is a possibility some might want some restrictions, e.g.:

• Parents and children can't gift each other
• Married people shouldn't be coupled together
• etc.

An open question for now:
Should this restriction be one-sided (a restriction from participant x to participant y but not the other way around - meaning participant y can still be chosen to be participant x secret Santa)? / should restrictions be applied both ways (participant x and participant y cannot gift each other)?

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The way to do this should be thoroughly thought through as it highly depends on the answer for the question above, and we want the functionality to reduce humanly errors as much as possible (e.g. solemnly relying on name basis could be a bit problematic as by requiring the retyping of other names fully and correctly there's a high chance of getting things wrong) - while not adding so much overhead that this would not be usable.