Account: organize allauth settings (#9865)

- make email verification mandatory: users without a verified email will be
  asked to verify their address before being able to login
- rename `ACTIVATION_DAYS` since it was a setting from a Django app we don't use
  anymore. It's called EMAIL_CONFIRMATION_EXPIRE_DAYS in Django Allauth.

This PR requires other PRs in -ops to work as intended.

Author: humitos

readthedocs/settings/base.py

@@ -710,11 +710,16 @@ def DOCKER_LIMITS(self):
     # All auth
     ACCOUNT_ADAPTER = 'readthedocs.core.adapters.AccountAdapter'
     ACCOUNT_EMAIL_REQUIRED = True
+
+    # Make email verification mandatory.
+    # Users won't be able to login until they verify the email address.
     ACCOUNT_EMAIL_VERIFICATION = 'mandatory'
+
     ACCOUNT_AUTHENTICATION_METHOD = 'username_email'
-    ACCOUNT_ACTIVATION_DAYS = 7
+    ACCOUNT_EMAIL_CONFIRMATION_EXPIRE_DAYS = 7
     SOCIALACCOUNT_AUTO_SIGNUP = False
     SOCIALACCOUNT_STORE_TOKENS = True
+
     SOCIALACCOUNT_PROVIDERS = {
         'github': {
             'SCOPE': [