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/private.go

@@ -11,6 +11,7 @@ import (
 
 	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/process"
+	"code.gitea.io/gitea/modules/web/middleware"
 )
 
 // PrivateContext represents a context for private routes
@@ -62,7 +63,7 @@ func PrivateContexter() func(http.Handler) http.Handler {
 			ctx := &PrivateContext{
 				Context: &Context{
 					Resp: NewResponse(w),
-					Data: map[string]interface{}{},
+					Data: middleware.GetContextData(req.Context()),
 				},
 			}
 			defer ctx.Close()