Add Bookmarking feature for users to follow and subscribe to other users

@cgruppioni @chukdisc
It's a bit of a thematic choice with the theme of the site, but I also want to eventually have users able to bookmark books (to keep tabs on any announcements/sales/etc that might have to do with them). Users also will have various different 'types' such as bookshops, authors, publishers so while 'following' might be the normal word to use, I think to bookmark accounts isn't too farfetched of an idea.