Load notification images as raw data from main process

[sirbrillig]

This PR attempts to solve #191 by fetching each user avatar image through the main process, using a proxy if configured for a particular account, then using Electron IPC to pass the raw image data to the renderer process and inject it dynamically into the DOM.

This uses the technique described in this SO answer: https://stackoverflow.com/a/77178841/1877316

To do

This works for the public images, but doesn't work for the proxied GitHub Enterprise server images and I'm not sure why. It seems that the problem might be the fetching itself; the web server looks like it might be returning a generic "image not found" image for each one.

Ah, I think it's because not only do the images require being proxied, they require having a logged-in session to the http server for the Enterprise instance! I wonder if we can somehow use Octokit to fetch the images?

[sirbrillig]

Some possible leads I need to follow:

• https://stackoverflow.com/questions/9240961/github-jsonp-source-code-api
• Get binary data from Content API with raw media type octokit/rest.js#14

[sirbrillig]

It's possible this may not be something we can do:

• How to fetch the Github Enterprise Avatar? octokit/octokit.js#2061