feat: resize favicons before storing them

Some websites are using images of O(10kB) when not )O(100kB) for their
favicons. As miniflux only displays them with a 16x16 resolution, let's do our
best to resize them before storing them in the database. This should make
miniflux consume less bandwidth when serving pages, for the joy of mobile users
on a small data plan.

Of course, images that already are 16x16 aren't resized.

internal/reader/icon/finder_test.go

@@ -4,8 +4,13 @@
 package icon // import "miniflux.app/v2/internal/reader/icon"
 
 import (
+	"bytes"
+	"encoding/base64"
+	"image"
 	"strings"
 	"testing"
+
+	"miniflux.app/v2/internal/model"
 )
 
 func TestParseImageDataURL(t *testing.T) {
@@ -125,3 +130,52 @@ func TestParseDocumentWithWhitespaceIconURL(t *testing.T) {
 		t.Errorf(`Invalid icon URL, got %q`, iconURLs[0])
 	}
 }
+
+func TestResizeIconSmallGif(t *testing.T) {
+	data, err := base64.StdEncoding.DecodeString("R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==")
+	if err != nil {
+		t.Fatal(err)
+	}
+	icon := model.Icon{
+		Content:  data,
+		MimeType: "image/gif",
+	}
+	if !bytes.Equal(icon.Content, resizeIcon(&icon).Content) {
+		t.Fatalf("Converted gif smaller than 16x16")
+	}
+}
+
+func TestResizeIconPng(t *testing.T) {
+	data, err := base64.StdEncoding.DecodeString("iVBORw0KGgoAAAANSUhEUgAAABEAAAARCAYAAAA7bUf6AAAAHElEQVR42mP8z/C/noFCwDhqyKgho4aMGkIlQwBrHSpf28Yx+gAAAABJRU5ErkJggg==")
+	if err != nil {
+		t.Fatal(err)
+	}
+	icon := model.Icon{
+		Content:  data,
+		MimeType: "image/png",
+	}
+	resizedIcon := resizeIcon(&icon)
+
+	if bytes.Equal(data, resizedIcon.Content) {
+		t.Fatalf("Didn't convert png of 17x17")
+	}
+
+	config, _, err := image.DecodeConfig(bytes.NewReader(resizedIcon.Content))
+	if err != nil {
+		t.Fatalf("Couln't decode resulting png: %v", err)
+	}
+
+	if config.Height != 16 || config.Width != 16 {
+		t.Fatalf("Was expecting an image of 16x16, got %dx%d", config.Width, config.Height)
+	}
+}
+
+func TestResizeInvalidImage(t *testing.T) {
+	icon := model.Icon{
+		Content:  []byte("invalid data"),
+		MimeType: "image/gif",
+	}
+	if !bytes.Equal(icon.Content, resizeIcon(&icon).Content) {
+		t.Fatalf("Tried to convert an invalid image")
+	}
+}