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/test/context_tests.go

@@ -30,7 +30,7 @@ func MockContext(t *testing.T, path string) *context.Context {
 	resp := &mockResponseWriter{}
 	ctx := context.Context{
 		Render: &mockRender{},
-		Data:   make(map[string]interface{}),
+		Data:   make(middleware.ContextData),
 		Flash: &middleware.Flash{
 			Values: make(url.Values),
 		},