Menus - Revise how contribution ordering works

[tjcouch-sil]

User Story
As a user, I want relatively stable menus so that I know where to find things.

As an extension developer, I want my menu contributions to show up properly so I can show users things.

Description
Right now, menu ordering is challenging because order must be unique. We are already struggling to avoid conflicts between items, and this will probably continue to get worse with groups and columns as Platform.Bible gets bigger. Let's find a solution to this problem that keeps menus stable and predictable yet avoids conflicts.

See this PR discussion for more information and ideas.