Move migrations into services and base into modules/migration (#17663)

* Move migrtions into services and base into modules/migration

* Fix imports

* Fix lint

modules/migration/release.go

@@ -0,0 +1,40 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migration
+
+import (
+	"io"
+	"time"
+)
+
+// ReleaseAsset represents a release asset
+type ReleaseAsset struct {
+	ID            int64
+	Name          string
+	ContentType   *string `yaml:"content_type"`
+	Size          *int
+	DownloadCount *int `yaml:"download_count"`
+	Created       time.Time
+	Updated       time.Time
+	DownloadURL   *string `yaml:"download_url"`
+	// if DownloadURL is nil, the function should be invoked
+	DownloadFunc func() (io.ReadCloser, error) `yaml:"-"`
+}
+
+// Release represents a release
+type Release struct {
+	TagName         string `yaml:"tag_name"`
+	TargetCommitish string `yaml:"target_commitish"`
+	Name            string
+	Body            string
+	Draft           bool
+	Prerelease      bool
+	PublisherID     int64  `yaml:"publisher_id"`
+	PublisherName   string `yaml:"publisher_name"`
+	PublisherEmail  string `yaml:"publisher_email"`
+	Assets          []*ReleaseAsset
+	Created         time.Time
+	Published       time.Time
+}