Add option to by default collapse the member list of Project-Overview

[tainewoo]

If the project contains a huge amount of members, the project overview page will become too long.

[alexandermeindl]

Keeping this open as an enhancement, with the analysis so far.

The real pain isn't only the list length but performance and clutter from group expansion: when a group with many users (e.g. 400) is added to a project, Redmine creates an inherited membership per user, so the member box lists the group and all its individual members - often across several roles.

"Collapse by default" only hides the symptom. The better direction is to show the group instead of its individually-inherited members in the member box.

Constraints we identified:

• This must stay display-only. The underlying Project#principals_by_role is also used by logic (e.g. manager-role resolution needs the expanded users), so collapsing groups in the model would break that. The fix belongs in a member-box display helper, not the model.
• Group names are already shown today for directly-assigned groups, so collapsing exposes nothing new. Whether group names should be shown at all is a separate question and would be its own option.

Likely a setting between two modes:

1. Current behavior (group + all inherited members)
2. Group instead of its inherited members (compact)

No timeline yet, but this captures the direction for whoever picks it up.