Consolidate similar menu locations strings

[DrewAPicture]

In the new 'Manage Locations' tab added to the Menus screen in 3.6, we have this plural/singular string:

'Your theme supports %s menu. Select which menu you would like to use below.', 'Your theme supports %s menus. Select a menu to use for each theme location below.'

And in the nav menus Customizer control we have this plural/singular string:

'Your theme supports %s menu. Select which menu you would like to use.', 'Your theme supports %s menus. Select which menu appears in each location.'

The patch consolidates both to use the Customizer control version.

[SergeyBiryukov]

In 25218:

Consolidate similar menu locations strings. props DrewAPicture. fixes #24945.