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

models/release_test.go

@@ -107,6 +107,7 @@ func TestRelease_MirrorDelete(t *testing.T) {
 		IsPrivate:   false,
 		IsMirror:    true,
 		RemoteAddr:  repoPath,
+		Wiki:        true,
 	}
 	mirror, err := MigrateRepository(user, user, migrationOptions)
 	assert.NoError(t, err)