Add an upgrade path from Admin Menu to Admin bar

from @klonos in https://github.com/backdrop/backdrop-issues/issues/3680#issuecomment-484...

Re admin_menu, doesn't it provide (some of) the same options as the core admin_bar module ("sticky at top of pages" etc.)? We should be migrating these settings over ...if not already doing so.

Reply from me:

I think that's the only setting that's the same between contrib and core, though it wouldn't hurt to check. I'll create an issue for the checking!

TODO: * check all the variables set in admin_menu * ensure any that are the same are upgraded in admin_bar * document those that are not provided in admin_bar * update the documentation at https://backdropcms.org/project/admin_menu