Ability to display static_placeholder on all sites

[beniwohli]

In c5b8c6a, a non-nullable FK to Site was introduced for static placeholders, to allow different content on different sites. But it also made a different, valid use case impossible: show the same content on multiple sites.

What about making the site FK nullable, and introduce a new flag to static_placeholder (e.g. "global") to allow static placeholders that are displayed on all sites?

I think the use case of displaying them on some, but not all sites is too much of a fringe case, and it would be difficult to find a UI that is understandable for the user.