Altered social auto-reg to be configurable per service

- Added {$service}_AUTO_REGISTER and {$service}_AUTO_CONFIRM_EMAIL env
options for each social auth system.
- Auto-register will allow registration from login, even if registration
is disabled.
- Auto-confirm-email indicates trust and will mark new registrants as
'email_confirmed' and skip 'confirmation email' flow.
- Also added covering tests.

app/Exceptions/SocialSignInAccountNotUsed.php

@@ -0,0 +1,6 @@
+<?php namespace BookStack\Exceptions;
+
+class SocialSignInAccountNotUsed extends SocialSignInException
+{
+
+}