Adds "Social Account Added" signal, code refactor

Logging in and signing up already have signals. I wanted one for the remaining case of doing a sociallogin, which is adding a social account. In the midst of doing this I refactored the code a little bit. `complete_social_login` seemed to call a helper function for the login and signup cases, I figured I'd do the same for adding a social account as well. (I also renamed "next" to "next_url" since "next" is a Python keyword.)

I've tried it out on my project, seems to work fine.