2.0: Proxied notifications do not show their images

[sirbrillig]

Since images are not being loaded through the proxy, they may fail to load for proxied accounts on GH Enterprise Servers.

[sirbrillig]

I tried to do this in #200 but it may not be possible to fetch an avatar image from GitHub Enterprise, not because of the proxy (I got that part working), but because of needing to be authenticated to the GH Enterprise instance to load images over HTTP. It appears that there's no way to use the GitHub (Enterprise) API to fetch avatar image data and I don't think there's a way to use the API key we have for the server to authenticate to the HTTP API.

I'm trying to think of other workarounds. If the user data had gravatar_id set, we could use that, but from what I can tell that field is never set on any data I get from the server.

We do have the user's username, and we could try to find the matching avatar on public GitHub, but the username might be different there (and I'm not sure how to map username to avatar there either). 🤔

[sirbrillig]

I suppose one workaround would be to make custom avatar images for each user and use those. It would be better than nothing. Trying that in #201