Font Awesome v5, discord link, RSS

I updated FA to the new version which includes a discord icon - it's now in the social section. I moved RSS to the blog since it was getting a bit crowded there and since that's where i'd look for it. Not sure if this is the correct way to do it, let me know if not.

Note that gitlab now also has an icon in FA, should we use it instead of the generic git one?

I made the git link point to the activity view since it looks more active ;) The group page just lists projects in a weird order and shows last update 1 month ago or longer for some reason.

I changed the class from fa to fab or fas since fa is deprecated. Some icons are slightly different now and some were renamed.