Users should be able to drag and drop an image into the editor to upload it.
When a user drags and drops a file into the editor:
* Start uploading
* Insert placeholder Markdown for the photo into the body of the post, e.g. `![filename.png](upload://filename.png)` like Discourse does
Once the upload finishes, replace e.g. `upload://filename.png` with the live photo URL.
If the upload fails, let's just remove the Markdown, as Discourse does. We want to be as unobtrusive as possible, so let's start with this and see how it turns out.
TODO: figure out backend implementation