Support PDF Uploads

[rajadain]

Overview

In addition to image files, users may upload PDFs as reference images. Currently, this is not supported by DistortableImage plugin for Leaflet.

We should decide what happens if there are multiple pages in the PDF file? Perhaps we can always add the first page? Answer these questions before starting the implementation.

Describe the solution you'd like

The user is able to select a PDF file and add it to the map.

Additional Context

To do this in the client may require a significant client-side library. A more reasonable option would be to upload to the back-end and have it return an image that can be added more easily.

See more here: #60 (comment)