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>

routers/api/v1/admin/user.go

@@ -103,7 +103,6 @@ func CreateUser(ctx *context.APIContext) {
 		if err != nil {
 			log.Error(err.Error())
 		}
-		ctx.Data["Err_Password"] = true
 		ctx.Error(http.StatusBadRequest, "PasswordPwned", errors.New("PasswordPwned"))
 		return
 	}
@@ -201,7 +200,6 @@ func EditUser(ctx *context.APIContext) {
 			if err != nil {
 				log.Error(err.Error())
 			}
-			ctx.Data["Err_Password"] = true
 			ctx.Error(http.StatusBadRequest, "PasswordPwned", errors.New("PasswordPwned"))
 			return
 		}