Allow per-backend user pipeline settings

According to
https://python-social-auth.readthedocs.io/en/latest/configuration/settings.html,
"All settings can be defined per-backend by adding the backend name to
the setting name, like SOCIAL_AUTH_TWITTER_LOGIN_URL". This changes
user.py to actually use per-backend settings for options like
USERNAME_IS_FULL_EMAIL.

Note that get_username is always called with a backend, but user_details
isn't, so a different version of `setting' is needed for the two.

social_core/pipeline/user.py

@@ -11,17 +11,17 @@ def get_username(strategy, details, backend, user=None, *args, **kwargs):
     storage = strategy.storage
 
     if not user:
-        email_as_username = strategy.setting('USERNAME_IS_FULL_EMAIL', False)
-        uuid_length = strategy.setting('UUID_LENGTH', 16)
+        email_as_username = backend.setting('USERNAME_IS_FULL_EMAIL', False)
+        uuid_length = backend.setting('UUID_LENGTH', 16)
         max_length = storage.user.username_max_length()
-        do_slugify = strategy.setting('SLUGIFY_USERNAMES', False)
-        do_clean = strategy.setting('CLEAN_USERNAMES', True)
+        do_slugify = backend.setting('SLUGIFY_USERNAMES', False)
+        do_clean = backend.setting('CLEAN_USERNAMES', True)
 
         def identity_func(val):
             return val
 
         if do_clean:
-            override_clean = strategy.setting('CLEAN_USERNAME_FUNCTION')
+            override_clean = backend.setting('CLEAN_USERNAME_FUNCTION')
             if override_clean:
                 clean_func = module_member(override_clean)
             else:
@@ -30,7 +30,7 @@ def identity_func(val):
             clean_func = identity_func
 
         if do_slugify:
-            override_slug = strategy.setting('SLUGIFY_FUNCTION')
+            override_slug = backend.setting('SLUGIFY_FUNCTION')
             if override_slug:
                 slug_func = module_member(override_slug)
             else:
@@ -88,19 +88,19 @@ def user_details(strategy, details, backend, user=None, *args, **kwargs):
 
     # Default protected user fields (username, id, pk and email) can be ignored
     # by setting the SOCIAL_AUTH_NO_DEFAULT_PROTECTED_USER_FIELDS to True
-    if strategy.setting('NO_DEFAULT_PROTECTED_USER_FIELDS') is True:
+    if strategy.setting('NO_DEFAULT_PROTECTED_USER_FIELDS', backend=backend) is True:
         protected = ()
     else:
         protected = ('username', 'id', 'pk', 'email', 'password',
                      'is_active', 'is_staff', 'is_superuser',)
 
-    protected = protected + tuple(strategy.setting('PROTECTED_USER_FIELDS', []))
+    protected = protected + tuple(strategy.setting('PROTECTED_USER_FIELDS', [], backend=backend))
 
     # Update user model attributes with the new data sent by the current
     # provider. Update on some attributes is disabled by default, for
     # example username and id fields. It's also possible to disable update
     # on fields defined in SOCIAL_AUTH_PROTECTED_USER_FIELDS.
-    field_mapping = strategy.setting('USER_FIELD_MAPPING', {}, backend)
+    field_mapping = strategy.setting('USER_FIELD_MAPPING', {}, backend=backend)
     for name, value in details.items():
         # Convert to existing user field if mapping exists
         name = field_mapping.get(name, name)
@@ -111,7 +111,7 @@ def user_details(strategy, details, backend, user=None, *args, **kwargs):
         if current_value == value:
             continue
 
-        immutable_fields = tuple(strategy.setting('IMMUTABLE_USER_FIELDS', []))
+        immutable_fields = tuple(strategy.setting('IMMUTABLE_USER_FIELDS', [], backend=backend))
         if name in immutable_fields and current_value:
             continue