Issue #2948793 by wizonesolutions, Liam Morland, liquidcms, noahzenzen: Support PDF security in pdftk

Issue #3046530 by Pancho, wizonesolutions: Revert: Add experimental sub-module to switch between BC and Next API

Issue #3040900 by Pancho: Move legacy backend support incl. the JavaBridge ("local") backend to a submodule

Issue #3046419 by Pancho, mondrake, wizonesolutions: Set locale to support filenames with non-ASCII characters in pdftk

Followup to: Issue #3046765 by Pancho, wizonesolutions: Discourage public:// scheme for storing populated PDFs

Snuck in a couple other fixes.

This gets us file.usage tracking for free.

I've been doing it wrong and a couple file fields need to be actual file fields in order
to track file usage properly and automatically.

Works for FillPDF forms now but not for generated PDFs.

There's lots of changes here. I should have committed more often.
Anyway, the gist of it is:
  - General code conversion to Drupal 8 patterns. I've taken shortcuts here and there but mostly avoid that. No idea on the quality. Hopefully, someone can give me some feedback.
  - Two new entity types, FillPdfForm and FillPdfFormField. These
    correspond to the old {fillpdf_forms} and {fillpdf_fields}
    tables. Now they're finally entities, so Views support will
    be possible for those of you who wanted it. I was finally forced to
    do this, and I'm glad.
  - PDF-filling methods are now PLUGINS!!! This means they all
    implement FillPdfBackendPluginInterface, and this basically
    achieves the same thing I was trying to do with the failed
    pdf_forms project. Anyone who wants to add their own
    backend can already do it; the fillpdf_service_backend
    configuration variable is used to select the correct backend.
    It should match the annotation ID. Look at
    FillPdfServiceFillPdfBackend to get an idea of how to structure the
    plugin.
  - Better use of actual entity references. FillPdfForms store a
    reference to their underlying Files instead of just an ID. Same with
    FillPdfFormField -> FillPdfForm.

This works with Webform nodes as the node as well.