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

docs/content/doc/advanced/migrations.en-us.md

@@ -0,0 +1,72 @@
+---
+date: "2019-04-15T17:29:00+08:00"
+title: "Advanced: Migrations Interfaces"
+slug: "migrations-interfaces"
+weight: 30
+toc: true
+draft: false
+menu:
+  sidebar:
+    parent: "advanced"
+    name: "Migrations Interfaces"
+    weight: 55
+    identifier: "migrations-interfaces"
+---
+
+# Migration Features
+
+The new migration features were introduced in Gitea 1.9.0. It defines two interfaces to support migrating 
+repositories data from other git host platforms to gitea or, in the future migrating gitea data to other 
+git host platforms. Currently, only the migrations from github via APIv3 to Gitea is implemented.
+
+First of all, Gitea defines some standard objects in packages `modules/migrations/base`. They are
+ `Repository`, `Milestone`, `Release`, `Label`, `Issue`, `Comment`, `PullRequest`.
+
+## Downloader Interfaces
+
+To migrate from a new git host platform, there are two steps to be updated.
+
+- You should implement a `Downloader` which will get all kinds of repository informations.
+- You should implement a `DownloaderFactory` which is used to detect if the URL matches and 
+create a Downloader.
+- You'll need to register the `DownloaderFactory` via `RegisterDownloaderFactory` on init.
+
+```Go
+type Downloader interface {
+	GetRepoInfo() (*Repository, error)
+	GetMilestones() ([]*Milestone, error)
+	GetReleases() ([]*Release, error)
+	GetLabels() ([]*Label, error)
+	GetIssues(start, limit int) ([]*Issue, error)
+	GetComments(issueNumber int64) ([]*Comment, error)
+	GetPullRequests(start, limit int) ([]*PullRequest, error)
+}
+```
+
+```Go
+type DownloaderFactory interface {
+	Match(opts MigrateOptions) (bool, error)
+	New(opts MigrateOptions) (Downloader, error)
+}
+```
+
+## Uploader Interface
+
+Currently, only a `GiteaLocalUploader` is implemented, so we only save downloaded 
+data via this `Uploader` on the local Gitea instance. Other uploaders are not supported
+and will be implemented in future.
+
+```Go
+// Uploader uploads all the informations
+type Uploader interface {
+	CreateRepo(repo *Repository, includeWiki bool) error
+	CreateMilestone(milestone *Milestone) error
+	CreateRelease(release *Release) error
+	CreateLabel(label *Label) error
+	CreateIssue(issue *Issue) error
+	CreateComment(issueNumber int64, comment *Comment) error
+	CreatePullRequest(pr *PullRequest) error
+	Rollback() error
+}
+
+```