Menus: have main menu assignment persist between theme switches

[kwight]

When a user switches to a new (never before used) theme, can we somehow assign their previous main menu to the new theme's main location? If we were to standardize on a register_nav_menu location key (say, primary), shouldn't we be able to:

• Check if the new theme has a location called primary
• If a menu is already assigned, use that one (the case when a user switches back to a previously used theme)
• If not, take the last theme's menu assigned to primary and assign it to the new theme's primary location
• If there's no primary location but only one location in the new theme, assign the original location to the only new theme location
• If there's no primary location but many menus, assign it to the first in the array of registered menus

[SergeyBiryukov]

Related: #17979 (for widgets).

[celloexpressions]

This is a duplicate of #18588, although the title and description here are a bit clearer.