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

modules/migrations/base/pullrequest.go

@@ -0,0 +1,53 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Copyright 2018 Jonas Franz. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"fmt"
+	"time"
+)
+
+// PullRequest defines a standard pull request information
+type PullRequest struct {
+	Number         int64
+	Title          string
+	PosterName     string
+	PosterEmail    string
+	Content        string
+	Milestone      string
+	State          string
+	Created        time.Time
+	Closed         *time.Time
+	Labels         []*Label
+	PatchURL       string
+	Merged         bool
+	MergedTime     *time.Time
+	MergeCommitSHA string
+	Head           PullRequestBranch
+	Base           PullRequestBranch
+	Assignee       string
+	Assignees      []string
+	IsLocked       bool
+}
+
+// IsForkPullRequest returns true if the pull request from a forked repository but not the same repository
+func (p *PullRequest) IsForkPullRequest() bool {
+	return p.Head.RepoPath() != p.Base.RepoPath()
+}
+
+// PullRequestBranch represents a pull request branch
+type PullRequestBranch struct {
+	CloneURL  string
+	Ref       string
+	SHA       string
+	RepoName  string
+	OwnerName string
+}
+
+// RepoPath returns pull request repo path
+func (p PullRequestBranch) RepoPath() string {
+	return fmt.Sprintf("%s/%s", p.OwnerName, p.RepoName)
+}