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>

modules/convert/pull.go

@@ -73,6 +73,8 @@ func ToAPIPullRequest(ctx context.Context, pr *models.PullRequest, doer *user_mo
 		Created:   pr.Issue.CreatedUnix.AsTimePtr(),
 		Updated:   pr.Issue.UpdatedUnix.AsTimePtr(),
 
+		AllowMaintainerEdit: pr.AllowMaintainerEdit,
+
 		Base: &api.PRBranchInfo{
 			Name:       pr.BaseBranch,
 			Ref:        pr.BaseBranch,