Allow editing Challenges

This feature cannot be implemented until issue 14 of harmonherring/CTF-API is complete.

It should be possible to edit a challenge once it's been created. When the creator visits the Challenge page, an edit button should appear somewhere on the page. Upon clicking this button, the user should be able to edit the aforementioned fields. Whether this is presented as a modal, the fields become editable on the page, or something else, just make sure it looks good and is easy to use. ONLY the changed fields should be PATCHed to `/challenges/` on the API.