Drag and Drop Image Upload

Combined with #2 and #3, this provides a simple drag and drop interface for uploading images [to imgur] and automatically inserting the correct image link to the message box for submission.

**Please note** that I'm using an API key here that I got from @achudars, and I am uncertain of how to acquire a key that works (as opposed to the (new?) OAuth2 `client_id` and `token`).  I'd lean towards leaving this pull request open until 1) we've resolved the API key issue, and 2) come up with a more intuitive UI for the upload process.

**TODO (maybe?)**
- [ ] API key issue resolution
- [ ] visual indicator of droppable state on drag-hover
- [ ] visual indicator of upload status (progress)