Take the social cover image for issue from the github repository of the editorial

[eliselavy]

Now:

need to be stored here:

https://github.com/C2DH/journal-of-digital-history/tree/develop/public/img/issues/social-media

should be same as article take into account the raw url on github

https://raw.githubusercontent.com/jdh-observer/jdh002-m734RWDSLo9C/main/socialmediacover.png