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>

modules/context/access_log.go

@@ -5,7 +5,6 @@ package context
 
 import (
 	"bytes"
-	"context"
 	"fmt"
 	"net"
 	"net/http"
@@ -13,8 +12,10 @@ import (
 	"text/template"
 	"time"
 
+	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/web/middleware"
 )
 
 type routerLoggerOptions struct {
@@ -26,8 +27,6 @@ type routerLoggerOptions struct {
 	RequestID      *string
 }
 
-var signedUserNameStringPointerKey interface{} = "signedUserNameStringPointerKey"
-
 const keyOfRequestIDInTemplate = ".RequestID"
 
 // According to:
@@ -60,8 +59,6 @@ func AccessLogger() func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 			start := time.Now()
-			identity := "-"
-			r := req.WithContext(context.WithValue(req.Context(), signedUserNameStringPointerKey, &identity))
 
 			var requestID string
 			if needRequestID {
@@ -73,9 +70,14 @@ func AccessLogger() func(http.Handler) http.Handler {
 				reqHost = req.RemoteAddr
 			}
 
-			next.ServeHTTP(w, r)
+			next.ServeHTTP(w, req)
 			rw := w.(ResponseWriter)
 
+			identity := "-"
+			data := middleware.GetContextData(req.Context())
+			if signedUser, ok := data[middleware.ContextDataKeySignedUser].(*user_model.User); ok {
+				identity = signedUser.Name
+			}
 			buf := bytes.NewBuffer([]byte{})
 			err = logTemplate.Execute(buf, routerLoggerOptions{
 				req:            req,