Add IP restriction in user import

[andreyv]

It would be enough to add a field in the user import form that applies the same value (subnet) to all participations being added.

[andreyv]

When a contest ends and analysis mode begins, we may also wish to remove user IP restriction. This could be implemented as a command line script that takes a contest id and updates its participations.

[karliss]

I was thinking of adding a form which allows group editing ip restriction for participant. That would work for adding/removing and changing it. It might even be upstreamed as it seems like convenient functionality for onsite contests.

[andreyv]

This also sounds like a simple contest-wide "IP restriction" setting.