Fix setting source url in uploader after fetch

[mdashlw]

Before you begin

• I understand my contributions may be rejected for any reason

• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software

• I understand my contributions are licensed under the GNU AGPLv3

• [x] I understand all of the above

---

js-image-source is a div that wraps the input and the delete button. actual input is js-source-url