Always include get_frontend_media() output, also for v1.0 format.

Excluding statically defined FrontendMedia if you return a
ContentItemOutput object makes no sense, and is only confusing.
Better have a consistent model, where get_frontend_media() is always
read. Since duplicate files are excluded, this should not affect anyone.
Small performance improvement to ImmutableMedia.__add__()
Protect against breaking frontend code by registering models to 2 plugins

This scenario isn't supported currently,
so better provide a hard check against it.