Feature: collate two single-sided multipage scans (#3784)

* Feature: collate two single-sided scans

Some ADF only support single-sided scans, making scanning
double-sided documents a bit annoying.

This new feature enables Paperless to do most of the work,
by merging two seperate scans into a single one, collating
the even and odd numbered pages.

* Documentation: clarify that collation is disabled by default

* Apply suggestions from code review

Co-authored-by: shamoon <4887959+shamoon@users.noreply.github.com>

* Address code review remarks

* Grammar fixes

---------

Co-authored-by: shamoon <4887959+shamoon@users.noreply.github.com>

src/documents/converters.py

@@ -0,0 +1,46 @@
+from pathlib import Path
+from subprocess import run
+
+import img2pdf
+from django.conf import settings
+from PIL import Image
+
+from documents.utils import copy_basic_file_stats
+
+
+def convert_from_tiff_to_pdf(tiff_path: Path, target_directory: Path) -> Path:
+    """
+    Converts a TIFF file into a PDF file.
+
+    The PDF will be created in the given target_directory and share the name of
+    the original TIFF file, as well as its stats (mtime etc.).
+
+    Returns the path of the PDF created.
+    """
+    with Image.open(tiff_path) as im:
+        has_alpha_layer = im.mode in ("RGBA", "LA")
+    if has_alpha_layer:
+        # Note the save into the temp folder, so as not to trigger a new
+        # consume
+        scratch_image = target_directory / tiff_path.name
+        run(
+            [
+                settings.CONVERT_BINARY,
+                "-alpha",
+                "off",
+                tiff_path,
+                scratch_image,
+            ],
+        )
+    else:
+        # Not modifying the original, safe to use in place
+        scratch_image = tiff_path
+
+    pdf_path = (target_directory / tiff_path.name).with_suffix(".pdf")
+
+    with scratch_image.open("rb") as img_file, pdf_path.open("wb") as pdf_file:
+        pdf_file.write(img2pdf.convert(img_file))
+
+    # Copy what file stat is possible
+    copy_basic_file_stats(tiff_path, pdf_path)
+    return pdf_path