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>

services/forms/repo_form.go

@@ -422,15 +422,16 @@ func (f *NewPackagistHookForm) Validate(req *http.Request, errs binding.Errors)
 
 // CreateIssueForm form for creating issue
 type CreateIssueForm struct {
-	Title       string `binding:"Required;MaxSize(255)"`
-	LabelIDs    string `form:"label_ids"`
-	AssigneeIDs string `form:"assignee_ids"`
-	Ref         string `form:"ref"`
-	MilestoneID int64
-	ProjectID   int64
-	AssigneeID  int64
-	Content     string
-	Files       []string
+	Title               string `binding:"Required;MaxSize(255)"`
+	LabelIDs            string `form:"label_ids"`
+	AssigneeIDs         string `form:"assignee_ids"`
+	Ref                 string `form:"ref"`
+	MilestoneID         int64
+	ProjectID           int64
+	AssigneeID          int64
+	Content             string
+	Files               []string
+	AllowMaintainerEdit bool
 }
 
 // Validate validates the fields
@@ -684,6 +685,11 @@ type DismissReviewForm struct {
 	Message  string
 }
 
+// UpdateAllowEditsForm form for changing if PR allows edits from maintainers
+type UpdateAllowEditsForm struct {
+	AllowMaintainerEdit bool
+}
+
 // __________       .__
 // \______   \ ____ |  |   ____ _____    ______ ____
 //  |       _// __ \|  | _/ __ \\__  \  /  ___// __ \