FGUs should be renamed to Group Conversations

Per @sytses, I'm changing how we refer to Functional Group Updates throughout the handbook as they're now going to be called, simply, Group Conversations (unabbreviated).

I've identified at a first pass 36 instances where the abbreviation FGU is used, including in one or more filenames, which means I'll need to ensure links to those pages continue to function after I've pushed the update. I'm marking this MR as a WIP as a result.

cc @Tknudsen