'Manage Locations' tab should be hidden for zero locations/no menus support

[DrewAPicture]

If you're working with a theme that doesn't have any registered menu locations, the 'Manage Locations' tab should be hidden. Patch attached.

[DrewAPicture]

24115.diff​ also fixes display of the 'This theme does not natively support menus' message based on number of theme locations rather than number of menus.

[DrewAPicture]

It's worth mentioning that if we don't hide the tab for this case, they'll visit the tab, it'll say 'Your theme supports 0 menus ...' and the list table will be empty, which is just bad UX.

[SergeyBiryukov]

In 24217:

Don't show "Manage Locations" tab if the theme has no menu locations defined. props DrewAPicture. fixes #24115.

[DrewAPicture]

24115.2.diff​ brings the no-support notice text more in line with the help tab, with the Widgets screen linked:

"Your theme does not natively support menus, but you can use them in sidebars by adding a Custom Menus widget on the ​Widgets screen."

[SergeyBiryukov]

In 24218:

Make the notice on Edit Menus screen more in line with the help text. props DrewAPicture. see #24115.