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/repo/issue/view_content/sidebar.tmpl

@@ -667,5 +667,21 @@
 				</form>
 			</div>
 		{{end}}
+
+		{{if and .Issue.IsPull .IsIssuePoster (not .Issue.IsClosed)}}
+			{{if and (not (eq .Issue.PullRequest.HeadRepo.FullName .Issue.PullRequest.BaseRepo.FullName)) .CanWriteToHeadRepo}}
+				<div class="ui divider"></div>
+				<div class="inline field">
+					<div class="ui checkbox" id="allow-edits-from-maintainers"
+							data-url="{{.Issue.Link}}"
+							data-prompt-tip="{{.i18n.Tr "repo.pulls.allow_edits_from_maintainers_desc"}}"
+							data-prompt-error="{{.i18n.Tr "repo.pulls.allow_edits_from_maintainers_err"}}"
+						>
+						<label><strong>{{.i18n.Tr "repo.pulls.allow_edits_from_maintainers"}}</strong></label>
+						<input type="checkbox" {{if .Issue.PullRequest.AllowMaintainerEdit}}checked{{end}}>
+					</div>
+				</div>
+			{{end}}
+		{{end}}
 	</div>
 </div>