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>

templates/swagger/v1_json.tmpl

@@ -14938,6 +14938,10 @@
       "description": "EditPullRequestOption options when modify pull request",
       "type": "object",
       "properties": {
+        "allow_maintainer_edit": {
+          "type": "boolean",
+          "x-go-name": "AllowMaintainerEdit"
+        },
         "assignee": {
           "type": "string",
           "x-go-name": "Assignee"
@@ -17045,6 +17049,10 @@
       "description": "PullRequest represents a pull request",
       "type": "object",
       "properties": {
+        "allow_maintainer_edit": {
+          "type": "boolean",
+          "x-go-name": "AllowMaintainerEdit"
+        },
         "assignee": {
           "$ref": "#/definitions/User"
         },