Redmine

This patch adds a new "Users by group" option to the "Assignee drop-down display format" setting introduced by #43996.

When selected, the assignee drop-down is structured as follows:

• Assignable groups are listed first uner a @Group" section.
• Users are then displayed in separate sections for each assignable group.
• Users who belong to multiple groups appear under each of those groups.
• Users that are not included in any of the displayed groups are shown separately under a final Users section.
• Users already listed under a group are not repeated in the "Users" section.

This structure makes it easier to locate assignees in projects with many members, especially when users are typically identified by their team or group.

This feature is inspired by the Redmine Assign Grouping plugin, which provided a useful way to organize users by group but is not compatible with recent Redmine versions. This patch brings a similar capability into core Redmine.