Empty shell in place of actor profile picture

Describe the bug

When remote actor doesn't have a profile picture, an empty shell is displayed.

Steps to reproduce

1. Reply to a Castopod note from Mastodon without a profile picture
2. Look at the reply on Castopod

Expected behavior

A default profile picture should be displayed instead.

Relevant logs and/or screenshots

Context

• Castopod: alpha.49
• OS: Ubuntu server
• Browser: Firefox
• Web server: Nginx

Possible fixes

If there is no profile pic, use a default one.