Improve migrations to support migrating milestones/labels/issues/comments/pullrequests (#6290)

* add migrations

* fix package dependency

* fix lints

* implements migrations except pull requests

* add releases

* migrating releases

* fix bug

* fix lint

* fix migrate releases

* fix tests

* add rollback

* pull request migtations

* fix import

* fix go module vendor

* add tests for upload to gitea

* more migrate options

* fix swagger-check

* fix misspell

* add options on migration UI

* fix log error

* improve UI options on migrating

* add support for username password when migrating from github

* fix tests

* remove comments and fix migrate limitation

* improve error handles

* migrate API will also support migrate milestones/labels/issues/pulls/releases

* fix tests and remove unused codes

* add DownloaderFactory and docs about how to create a new Downloader

* fix misspell

* fix migration docs

* Add hints about migrate options on migration page

* fix tests

vendor/github.com/google/go-github/v24/github/strings.go

@@ -0,0 +1,93 @@
+// Copyright 2013 The go-github AUTHORS. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package github
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+
+	"reflect"
+)
+
+var timestampType = reflect.TypeOf(Timestamp{})
+
+// Stringify attempts to create a reasonable string representation of types in
+// the GitHub library. It does things like resolve pointers to their values
+// and omits struct fields with nil values.
+func Stringify(message interface{}) string {
+	var buf bytes.Buffer
+	v := reflect.ValueOf(message)
+	stringifyValue(&buf, v)
+	return buf.String()
+}
+
+// stringifyValue was heavily inspired by the goprotobuf library.
+
+func stringifyValue(w io.Writer, val reflect.Value) {
+	if val.Kind() == reflect.Ptr && val.IsNil() {
+		w.Write([]byte("<nil>"))
+		return
+	}
+
+	v := reflect.Indirect(val)
+
+	switch v.Kind() {
+	case reflect.String:
+		fmt.Fprintf(w, `"%s"`, v)
+	case reflect.Slice:
+		w.Write([]byte{'['})
+		for i := 0; i < v.Len(); i++ {
+			if i > 0 {
+				w.Write([]byte{' '})
+			}
+
+			stringifyValue(w, v.Index(i))
+		}
+
+		w.Write([]byte{']'})
+		return
+	case reflect.Struct:
+		if v.Type().Name() != "" {
+			w.Write([]byte(v.Type().String()))
+		}
+
+		// special handling of Timestamp values
+		if v.Type() == timestampType {
+			fmt.Fprintf(w, "{%s}", v.Interface())
+			return
+		}
+
+		w.Write([]byte{'{'})
+
+		var sep bool
+		for i := 0; i < v.NumField(); i++ {
+			fv := v.Field(i)
+			if fv.Kind() == reflect.Ptr && fv.IsNil() {
+				continue
+			}
+			if fv.Kind() == reflect.Slice && fv.IsNil() {
+				continue
+			}
+
+			if sep {
+				w.Write([]byte(", "))
+			} else {
+				sep = true
+			}
+
+			w.Write([]byte(v.Type().Field(i).Name))
+			w.Write([]byte{':'})
+			stringifyValue(w, fv)
+		}
+
+		w.Write([]byte{'}'})
+	default:
+		if v.CanInterface() {
+			fmt.Fprint(w, v.Interface())
+		}
+	}
+}