Wiki images can't be downloaded when project is private

[BenitoMatischen]

To replicate

Create a private project. In its wiki add an image as a file stored on OSF Storage.
Attempt to export the project with the tool. You should get either a 401 error, or a PIL.UnidentifiedImageError error.

Wiki images stored on OSF/external links can be downloaded normally for public projects.

Example when images work:

Example when images don't work:

[BenitoMatischen]

As a workaround, PR #93 made it so for that any images we can't access, we instead insert a link to those images in the PDF.

[BenitoMatischen]

Code relevant for this issue is here

osf-project-exporter/src/osfexport/formatter.py

Line 15 in 6722b0f

class HTMLImageSizeCapRenderer(HTMLRenderer):