Improve template system and panic recovery (#24461)

Partially for #24457

Major changes:

1. The old `signedUserNameStringPointerKey` is quite hacky, use
`ctx.Data[SignedUser]` instead
2. Move duplicate code from `Contexter` to `CommonTemplateContextData`
3. Remove incorrect copying&pasting code `ctx.Data["Err_Password"] =
true` in API handlers
4. Use one unique `RenderPanicErrorPage` for panic error page rendering
5. Move `stripSlashesMiddleware` to be the first middleware
6. Install global panic recovery handler, it works for both `install`
and `web`
7. Make `500.tmpl` only depend minimal template functions/variables,
avoid triggering new panics

Screenshot:

<details>

![image](https://user-images.githubusercontent.com/2114189/235444895-cecbabb8-e7dc-4360-a31c-b982d11946a7.png)

</details>

routers/install/install.go

@@ -5,7 +5,6 @@
 package install
 
 import (
-	goctx "context"
 	"fmt"
 	"net/http"
 	"os"
@@ -53,33 +52,32 @@ func getSupportedDbTypeNames() (dbTypeNames []map[string]string) {
 	return dbTypeNames
 }
 
-// Init prepare for rendering installation page
-func Init(ctx goctx.Context) func(next http.Handler) http.Handler {
+// Contexter prepare for rendering installation page
+func Contexter() func(next http.Handler) http.Handler {
 	rnd := templates.HTMLRenderer()
 	dbTypeNames := getSupportedDbTypeNames()
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
-			locale := middleware.Locale(resp, req)
-			startTime := time.Now()
 			ctx := context.Context{
 				Resp:    context.NewResponse(resp),
 				Flash:   &middleware.Flash{},
-				Locale:  locale,
+				Locale:  middleware.Locale(resp, req),
 				Render:  rnd,
+				Data:    middleware.GetContextData(req.Context()),
 				Session: session.GetSession(req),
-				Data: map[string]interface{}{
-					"locale":        locale,
-					"Title":         locale.Tr("install.install"),
-					"PageIsInstall": true,
-					"DbTypeNames":   dbTypeNames,
-					"AllLangs":      translation.AllLangs(),
-					"PageStartTime": startTime,
-
-					"PasswordHashAlgorithms": hash.RecommendedHashAlgorithms,
-				},
 			}
 			defer ctx.Close()
 
+			ctx.Data.MergeFrom(middleware.CommonTemplateContextData())
+			ctx.Data.MergeFrom(middleware.ContextData{
+				"locale":        ctx.Locale,
+				"Title":         ctx.Locale.Tr("install.install"),
+				"PageIsInstall": true,
+				"DbTypeNames":   dbTypeNames,
+				"AllLangs":      translation.AllLangs(),
+
+				"PasswordHashAlgorithms": hash.RecommendedHashAlgorithms,
+			})
 			ctx.Req = context.WithContext(req, &ctx)
 			next.ServeHTTP(resp, ctx.Req)
 		})