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/request.go

@@ -12,8 +12,3 @@ import (
 func IsAPIPath(req *http.Request) bool {
 	return strings.HasPrefix(req.URL.Path, "/api/")
 }
-
-// IsInternalPath returns true if the specified URL is an internal API path
-func IsInternalPath(req *http.Request) bool {
-	return strings.HasPrefix(req.URL.Path, "/api/internal/")
-}