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, 27 insertions, 5 deletions

diff --git a/modules/structs/pull.go b/modules/structs/pull.go
index 653091b2f4..b63b3edfd3 100644
--- a/modules/structs/pull.go
+++ b/modules/structs/pull.go
@@ -31,9 +31,10 @@ type PullRequest struct {
 	Mergeable bool `json:"mergeable"`
 	HasMerged bool `json:"merged"`
 	// swagger:strfmt date-time
-	Merged         *time.Time `json:"merged_at"`
-	MergedCommitID *string    `json:"merge_commit_sha"`
-	MergedBy       *User      `json:"merged_by"`
+	Merged              *time.Time `json:"merged_at"`
+	MergedCommitID      *string    `json:"merge_commit_sha"`
+	MergedBy            *User      `json:"merged_by"`
+	AllowMaintainerEdit bool       `json:"allow_maintainer_edit"`
 
 	Base      *PRBranchInfo `json:"base"`
 	Head      *PRBranchInfo `json:"head"`
@@ -90,6 +91,7 @@ type EditPullRequestOption struct {
 	Labels    []int64  `json:"labels"`
 	State     *string  `json:"state"`
 	// swagger:strfmt date-time
-	Deadline       *time.Time `json:"due_date"`
-	RemoveDeadline *bool      `json:"unset_due_date"`
+	Deadline            *time.Time `json:"due_date"`
+	RemoveDeadline      *bool      `json:"unset_due_date"`
+	AllowMaintainerEdit *bool      `json:"allow_maintainer_edit"`
 }
diff --git a/modules/structs/repo_file.go b/modules/structs/repo_file.go
index e2947bf7ac..135e6484cd 100644
--- a/modules/structs/repo_file.go
+++ b/modules/structs/repo_file.go
@@ -30,6 +30,11 @@ type CreateFileOptions struct {
 	Content string `json:"content"`
 }
 
+// Branch returns branch name
+func (o *CreateFileOptions) Branch() string {
+	return o.FileOptions.BranchName
+}
+
 // DeleteFileOptions options for deleting files (used for other File structs below)
 // Note: `author` and `committer` are optional (if only one is given, it will be used for the other, otherwise the authenticated user will be used)
 type DeleteFileOptions struct {
@@ -39,6 +44,11 @@ type DeleteFileOptions struct {
 	SHA string `json:"sha" binding:"Required"`
 }
 
+// Branch returns branch name
+func (o *DeleteFileOptions) Branch() string {
+	return o.FileOptions.BranchName
+}
+
 // UpdateFileOptions options for updating files
 // Note: `author` and `committer` are optional (if only one is given, it will be used for the other, otherwise the authenticated user will be used)
 type UpdateFileOptions struct {
@@ -50,6 +60,16 @@ type UpdateFileOptions struct {
 	FromPath string `json:"from_path" binding:"MaxSize(500)"`
 }
 
+// Branch returns branch name
+func (o *UpdateFileOptions) Branch() string {
+	return o.FileOptions.BranchName
+}
+
+// FileOptionInterface provides a unified interface for the different file options
+type FileOptionInterface interface {
+	Branch() string
+}
+
 // ApplyDiffPatchFileOptions options for applying a diff patch
 // Note: `author` and `committer` are optional (if only one is given, it will be used for the other, otherwise the authenticated user will be used)
 type ApplyDiffPatchFileOptions struct {