Add Bluesky logo to site footer

[eloquence]

Since we now have an official Bluesky account (https://bsky.app/profile/securedrop.org), we should add it to the site footer as well. Let's update the bird logo as well while we're there.

[zenmonkeykstop]

Can we leave the bird logo as is? I'm about as inclined to have the X logo there as I would be to have the Gab one.

[eloquence]

As you know, I'd prefer not to have a presence there at all, but if we do, I would prefer to be consistent (we've updated it on our other sites). That bird has been dead for a while.

But we can scope it out of this issue for now and maybe resolve it together with final decision on continued presence there.