enhance(admin): show impersonation banner and keep password change with the user (#38924)

Follow-up to https://github.com/go-gitea/gitea/pull/38614

---------

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

services/context/context_template.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"time"
 
+	user_model "gitea.dev/models/user"
 	"gitea.dev/modules/htmlutil"
 	"gitea.dev/modules/httplib"
 	"gitea.dev/modules/public"
@@ -66,6 +67,14 @@ func (c TemplateContext) CurrentWebTheme() *webtheme.ThemeMetaInfo {
 	return webtheme.GuaranteeGetThemeMetaInfo(themeName)
 }
 
+func (c TemplateContext) ImpersonatedUser() *user_model.User {
+	webCtx := GetWebContext(c)
+	if webCtx == nil || webCtx.Doer == nil || !webCtx.DoerIsImpersonated() {
+		return nil
+	}
+	return webCtx.Doer
+}
+
 func (c TemplateContext) CurrentWebBanner() *setting.WebBannerType {
 	// Using revision as a simple approach to determine if the banner has been changed after the user dismissed it.
 	// There could be some false-positives because revision can be changed even if the banner isn't.