Allow data-entity-substitution to use configured plugin for media matchers.

I had forgotten about this, apologies for the lengthy review time. This is good for RTBC, tested against a custom substitution plugin with intended results. Thanks for the tests.

mmaranao → changed the visibility of the branch 3299988-allow-data-entity-substitution-to to active.

smustgrave → closed merge request !3

Pipeline needs a good amount of work and needs to work with ck5 but this seems good.