Add "Allow edits from maintainer" feature (#18002)

Adds a feature [like GitHub has](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) (step 7).
If you create a new PR from a forked repo, you can select (and change later, but only if you are the PR creator/poster) the "Allow edits from maintainers" option.
Then users with write access to the base branch get more permissions on this branch:
* use the update pull request button
* push directly from the command line (`git push`)
* edit/delete/upload files via web UI
* use related API endpoints

You can't merge PRs to this branch with this enabled, you'll need "full" code write permissions.

This feature has a pretty big impact on the permission system. I might forgot changing some things or didn't find security vulnerabilities. In this case, please leave a review or comment on this PR.

Closes #17728

Co-authored-by: 6543 <6543@obermui.de>

2 files changed, 21 insertions, 0 deletions

diff --git a/models/migrations/migrations.go b/models/migrations/migrations.go
index ba8ca85bc8..9e46791ec6 100644
--- a/models/migrations/migrations.go
+++ b/models/migrations/migrations.go
@@ -61,6 +61,7 @@ type Version struct {
 // update minDBVersion accordingly
 var migrations = []Migration{
 	// Gitea 1.5.0 ends at v69
+
 	// v70 -> v71
 	NewMigration("add issue_dependencies", addIssueDependencies),
 	// v71 -> v72
@@ -380,6 +381,8 @@ var migrations = []Migration{
 	NewMigration("Create ForeignReference table", createForeignReferenceTable),
 	// v212 -> v213
 	NewMigration("Add package tables", addPackageTables),
+	// v213 -> v214
+	NewMigration("Add allow edits from maintainers to PullRequest table", addAllowMaintainerEdit),
 }
 
 // GetCurrentDBVersion returns the current db version
diff --git a/models/migrations/v213.go b/models/migrations/v213.go
new file mode 100644
index 0000000000..b1dbf98d1e
--- /dev/null
+++ b/models/migrations/v213.go
@@ -0,0 +1,18 @@
+// Copyright 2022 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 (
+	"xorm.io/xorm"
+)
+
+func addAllowMaintainerEdit(x *xorm.Engine) error {
+	// PullRequest represents relation between pull request and repositories.
+	type PullRequest struct {
+		AllowMaintainerEdit bool `xorm:"NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync2(new(PullRequest))
+}