Django admin for Store model: M2M field for store_whitelist and snap_inclusion_store_whitelist are error prone

[Natalia Bidart]

When managing a Store instance via the Django admin, the M2M relationships store_whitelist and snap_inclusion_store_whitelist are "plain multi-select widgets" which at the moment they contain around 300 stores each. This has the negative consequences of:

1- looking for a specific store is difficult, since there is not filter capability (though right now stores are sorted by name, so that helps a bit).

2- when selecting more than one store, the user has to be sure about what they are doing since being a multi-select widget, CTRL has to be used to not deselect previous selections.

Ideally, these two fields widgets would be replaced by the same widgets that the Django admin shows for users, for when handling groups and permissions, where a filter text entry is provided, and the multi-selection is done by moving choices from left to right between the "not selected" and "selected" boxes.