* Store original author info for migrated issues and comments

Keep original author name for displaying in Gitea interface and also
store original author user ID for potential future use in linking
accounts from old location.

* Add original_url for repo

Store the original URL for a migrated repo

Clean up migrations/tests

* fix migration

* fix golangci-lint

* make 'make revive' happy also

* Modify templates to use OriginalAuthor if set

Use the original author name in templates if it is set rather than the
user who migrated/currently owns the issues

* formatting fixes

* make generate-swagger

* Use default avatar for imported comments

* Remove no longer used IgnoreIssueAuthor option

* Add OriginalAuthorID to swagger also

* 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