refactor: move go-chi/captcha into Gitea (#39529)

The `gitea.com/go-chi/captcha` package only exists for Gitea,
so it moves into `modules/imagecaptcha`.

- Reloading an expired challenge shows a new image instead of a broken one
- Every answer is consumed on its first check
- OpenID registration stops after a failed captcha

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/auth/openid.go

@@ -378,7 +378,9 @@ func RegisterOpenIDPost(ctx *context.Context) {
 			ctx.ServerError("", err)
 			return
 		}
-		context.VerifyCaptcha(ctx, tplSignUpOID, form)
+		if !context.VerifyCaptcha(ctx, tplSignUpOID, form) {
+			return
+		}
 	}
 
 	length := max(setting.MinPasswordLength, 256)