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/web/middleware/flash.go

@@ -18,7 +18,7 @@ var FlashNow bool
 
 // Flash represents a one time data transfer between two requests.
 type Flash struct {
-	DataStore
+	DataStore ContextDataStore
 	url.Values
 	ErrorMsg, WarningMsg, InfoMsg, SuccessMsg string
 }
@@ -34,7 +34,7 @@ func (f *Flash) set(name, msg string, current ...bool) {
 	}
 
 	if isShow {
-		f.GetData()["Flash"] = f
+		f.DataStore.GetData()["Flash"] = f
 	} else {
 		f.Set(name, msg)
 	}