Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/model/icon.go

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+import (
+	"encoding/base64"
+	"fmt"
+)
+
+// Icon represents a website icon (favicon)
+type Icon struct {
+	ID       int64  `json:"id"`
+	Hash     string `json:"hash"`
+	MimeType string `json:"mime_type"`
+	Content  []byte `json:"content"`
+}
+
+// DataURL returns the data URL of the icon.
+func (i *Icon) DataURL() string {
+	return fmt.Sprintf("%s;base64,%s", i.MimeType, base64.StdEncoding.EncodeToString(i.Content))
+}
+
+// Icons represents a list of icons.
+type Icons []*Icon
+
+// FeedIcon is a junction table between feeds and icons.
+type FeedIcon struct {
+	FeedID int64 `json:"feed_id"`
+	IconID int64 `json:"icon_id"`
+}