That feature will not be ready (or at least, not tested enough) to be
officially part of 2.80 beta. So we disable it by default, hidding it
behind a startup option (`--enable-static-override`), and a python
app var (`bpy.app.use_static_override`).
That way, people who really want to play with it can do it easily, while
not exposing/enabling non-production-ready feature by default.
Note that underlying override code remains active, i.e. files we do have
overridden data-blocks will be loaded correctly according to static override.
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