enhance(repo): allow custom content in issue and pull request sidebars (#39764)

Add custom template support for issue sidebar.

* Fix https://github.com/go-gitea/gitea/issues/39763

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Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/issue/view_content/sidebar.tmpl

@@ -1,4 +1,5 @@
 <div class="issue-content-right ui segment" data-global-init="initRepoIssueSidebar">
+	{{template "custom/extra_repo_issue_sidebar_top" $}}
 	{{template "repo/issue/branch_selector_field" $}}
 
 	{{if .Issue.IsPull}}