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

Issue #3046257 by Pancho, wizonesolutions: Move EntityHelper::getFormFields() to the actual FillPdfForm entity and deprecate the 'fillpdf.entity_helper' service

Issue #3040903 by Pancho: Offer all available file schemes, but only those, preselecting the site default

Issue #2828240 by pianomansam: BaseFieldDefinitions in FillPdfFormField are missing required "weight" display option

Snuck in a couple other fixes.

Also fixes file usage handling for FillPdfForm.

This gets us file.usage tracking for free.

And a lot more. Fleshed out some services.

This is the same concept as the fillpdf_file_context table in D7. It
lets the module
store generation info for generated PDFs so that private storage access
checking
can work properly. I'll be simplifying this a lot in D8 and simply
storing a
URL that I can parse to get the entities whose access needs to be
checked. And
I might backport that to D7; it's much simpler.

Works for FillPDF forms now but not for generated PDFs.

Still more old-style syntax lurking around.

This reverts commit 49e2bf05.

This reverts commit fcfa48c0.