Remove unassigned bartender consumption form access with assign_tenders enabled

[FrankvanMourik]

In the current implementation, when the assign_tenders boolean is True for an organization, all bartenders (either assigned or unassigned) can see the buttons to open POS&DCF. The logic used for rendering this button is:

alexia/templates/scheduling/event_list.html

Line 128 in 9431a47

{% if e in events_tending or request.organization == e.organizer and request.organization.assigns_tenders and is_tender %}

We propose an improvement that only assigned bartenders and organization managers can see the buttons. So that would change the rendering logic to:
{% if e in events_tending or request.organization == e.organizer and request.organization.assigns_tenders and is_manager %} where is_manager checks whether the user is a manager of e.organizer.