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/git.go

@@ -0,0 +1,69 @@
+// 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 migrations
+
+import (
+	"code.gitea.io/gitea/modules/migrations/base"
+)
+
+var (
+	_ base.Downloader = &PlainGitDownloader{}
+)
+
+// PlainGitDownloader implements a Downloader interface to clone git from a http/https URL
+type PlainGitDownloader struct {
+	ownerName string
+	repoName  string
+	remoteURL string
+}
+
+// NewPlainGitDownloader creates a git Downloader
+func NewPlainGitDownloader(ownerName, repoName, remoteURL string) *PlainGitDownloader {
+	return &PlainGitDownloader{
+		ownerName: ownerName,
+		repoName:  repoName,
+		remoteURL: remoteURL,
+	}
+}
+
+// GetRepoInfo returns a repository information
+func (g *PlainGitDownloader) GetRepoInfo() (*base.Repository, error) {
+	// convert github repo to stand Repo
+	return &base.Repository{
+		Owner:    g.ownerName,
+		Name:     g.repoName,
+		CloneURL: g.remoteURL,
+	}, nil
+}
+
+// GetMilestones returns milestones
+func (g *PlainGitDownloader) GetMilestones() ([]*base.Milestone, error) {
+	return nil, ErrNotSupported
+}
+
+// GetLabels returns labels
+func (g *PlainGitDownloader) GetLabels() ([]*base.Label, error) {
+	return nil, ErrNotSupported
+}
+
+// GetReleases returns releases
+func (g *PlainGitDownloader) GetReleases() ([]*base.Release, error) {
+	return nil, ErrNotSupported
+}
+
+// GetIssues returns issues according start and limit
+func (g *PlainGitDownloader) GetIssues(start, limit int) ([]*base.Issue, error) {
+	return nil, ErrNotSupported
+}
+
+// GetComments returns comments according issueNumber
+func (g *PlainGitDownloader) GetComments(issueNumber int64) ([]*base.Comment, error) {
+	return nil, ErrNotSupported
+}
+
+// GetPullRequests returns pull requests according start and limit
+func (g *PlainGitDownloader) GetPullRequests(start, limit int) ([]*base.PullRequest, error) {
+	return nil, ErrNotSupported
+}