:bug: Use default Gravatar placeholder
:bug: Use default Gravatar placeholder

We had a custom one based on GitHub's default avatar, but it's down now.
As we haven't been using any GitHub API to display avatars,
it might be the best solution to just use the default Gravatar icon.
Also, we get one less external request that way.

chaosdorf/mete,YtvwlD/mete,chaosdorf/mete,YtvwlD/mete,YtvwlD/mete,chaosdorf/mete,chaosdorf/mete,YtvwlD/mete