cooperator_eater : resolve "eater" tab inconsistency on candidate cooperators

[polchampion]

As is, the "eater" tab appears on candidate cooperators BUT the share constraint on the max nbr of eaters is not inforced until the partner becomes effective cooperator.

I see two functional ways to solve this:
A. Remove the tab on candidate cooperators
B. Inforce eater constraint on candidate cooperators

Option A restricts flexibility.
Option B allows candidate cooperators to register eaters (which may not be a wanted feature at some cooperatives). However this is no different than currently, and was never reported, so this argument for option A shouldn't be considered.

Option B makes most sense to me.

[polchampion]

@victor-champonnois pour suivi