Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

resources/image.go

@@ -21,7 +21,6 @@ import (
 	"image/draw"
 	"image/jpeg"
 	"io"
-	"io/ioutil"
 	"os"
 	"strconv"
 	"strings"
@@ -126,8 +125,6 @@ type Image struct {
 	configInit   sync.Once
 	configLoaded bool
 
-	copyToDestinationInit sync.Once
-
 	imaging *Imaging
 
 	format imaging.Format
@@ -462,30 +459,23 @@ func (i *Image) decodeSource() (image.Image, error) {
 	return img, err
 }
 
+// returns an opened file or nil if nothing to write.
 func (i *Image) openDestinationsForWriting() (io.WriteCloser, error) {
 	targetFilenames := i.targetFilenames()
 	var changedFilenames []string
 
 	// Fast path:
-	// This is a processed version of the original.
-	// If it exists on destination with the same filename and file size, it is
-	// the same file, so no need to transfer it again.
+	// This is a processed version of the original;
+	// check if it already existis at the destination.
 	for _, targetFilename := range targetFilenames {
-		if fi, err := i.spec.BaseFs.PublishFs.Stat(targetFilename); err == nil && fi.Size() == i.osFileInfo.Size() {
+		if _, err := i.spec.BaseFs.PublishFs.Stat(targetFilename); err == nil {
 			continue
 		}
 		changedFilenames = append(changedFilenames, targetFilename)
 	}
 
 	if len(changedFilenames) == 0 {
-		return struct {
-			io.Writer
-			io.Closer
-		}{
-			ioutil.Discard,
-			ioutil.NopCloser(nil),
-		}, nil
-
+		return nil, nil
 	}
 
 	return helpers.OpenFilesForWriting(i.spec.BaseFs.PublishFs, changedFilenames...)