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
