Support for taking pictures of Rocq team members from somewhere else than GitHub

[Zimmi48]

Currently, Rocq team members have to be linked to a GitHub account, and this is where pictures are loaded from. In the future, some Rocq team members (in teams such as forum moderation, social media, Platform editorial board or members of working groups) could very well not have a GitHub account. We should support this case. It would also allow us to put back an "Alumni" team to list past members.