add status undecided frontière distinct

[boogheta]

Some users would like to tackle both usecases of the UNDECIDED status at the same time (both frontier of webentities included but uncrawled, and webentities unsure to include yet). For that usecase, having a 5th SEMI-IN status for the frontier would make sense, but it seems to me like this would require way too many changes in the whole design of both frontend and backend, so I'm more encline to consider this a won'tfix