Removed all traces of FFS from site (replaces !1027)

This is MR supersedes !1027 (closed), which was used as a base.

The main changes proposed by !1027 (closed) were already implemented during these months, so i used a different approach and directly removed the folder forum-funding-system, since i don't think it's necessary to keep a single file in a dedicated folder to communicate that section doesn't exist anymore. I also adapted the language files for it and fr.