generize form confirm warnings and overlapping plage ouvertures display

Created by: adipasquale

preliminary to https://github.com/betagouv/rdv-solidarites.fr/pull/987

• extract logic to filter plage ouvertures on departement scope from the plage_ouvertures controller to a generic view helper
• generize overlapping_plage_ouvertures partial so it accepts a random model duck-typed to respond to a overlapping_plage_ouvertures method
• extract the display overlapping warnings + Confirm or change values view from the specific plage ouverture form to the generic model_errors partial