Allow todos assigned to multiple people to be completed either by everyone or only once

When todos are assigned to multiple people, it must be marked as complete by each person for the overall todo to be completed. This is great for items where each person needs to independently take an action, but often we have a todo that represents a single action / piece of work that is going to be jointly done by multiple people. With those each person has to be marked as complete, which is confusing and might be missed.

Risk that todos assigned to multiple people are not properly marked as complete when they are in fact done. This is particularly a potential problem for occasional Aha users.

Todos should have a selection option so that either they have to be marked as done by each person or that the overall todo is marked as done only once (and then the ability to mark as done by person is removed). This could have defaults set at either the account or workspace level, or the default could stay that everyone has to mark the todo done as is the current functionality.