And some code style.

Now with real tokens again instead of the fake D7 token! We look at metdata
and stuff to figure out if it's an image field token! For great profit!

Snuck in a couple other fixes.

Turns out tokens don't get replaced with empty values anyway, so reversing
the replacement order was good enough!

And update the feature description.

So far, I have defined the interface. Need to actually implement the class
and move the logic from HandlePdfController.

Also fixes file usage handling for FillPdfForm.

A few things happened here. I split out the access-checking logic into a
FillPdfAccessHelper service. In coordination with that, I added
FillPdfLinkManipulator::parseUrlString() to facilitate parsing the
context strings we store in FillPdfFileContext objects. I also changed
the way I store URLs in FillPdfFileContext.context so that they are
easier to pass to Url::fromUri() for parsing (and I made a helper
method that parses them into a form Uri::fromUri() likes).

In short, saving PDFs to files now works as well as downloading. Score!

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.

And a lot more. Fleshed out some services.

This isn't really part of what link manipulation has to do. parseRequest is more or less
a helper function that should get the current request's Url and pass it to parseLink, which
then will do most of the actual work. This makes the service more reusable.

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.

The saving part is almost working. Need to get FillPDF file contexts
being saved properly. Then, I need to ensure that access
checking on the files works like I expect. I will probably have to
implement some hook or another.