add the ability to comment on artists, songs, albums, and user profiles

I haven't seen this brought up for a while, and I think it should be considered again. the ability to do this is the only reason I still use last.fm at all. i understand that moderation wouldn't be especially easy, but I think it'd add a lot to the app. if there is the ability to comment on other user profiles, I think you should be able to opt out of allowing this. adding the ability to block users (so you wouldn't see their comments anywhere) should be added alongside this if added. I think it would add a lot to the social aspect of the app, I think it'd be fun.