No-op edit_post_link() when in a customizer preview

[helen]

I saw a comment about how the combination of shift-click hint + visible edit shortcuts + disabled cursor over inaccessible links was confusing, and I tend to agree. It's also silly to have edit links that don't work and don't represent a preview except for privileged logged-in users, so I say we no-op them (in a way that can be undone by devs who actually have a use for them in the preview pane).

Related: #27403, #30937

[westonruter]

In 39133:

Customize: No-op edit_post_link() when in customizer preview.

Edit links can be restored in plugins by removing the __return_empty_string function from the get_edit_post_link filter at the customize_preview_init action.

Fixes #38648.