groups: Pass old setting value for can_mention_group.

This commit adds support to pass object containing both old and new
values of the can_mention_group setting, as well as detailed API
documentation for this part of the API system.

Co-authored-by: Tim Abbott <tabbott@zulip.com>
Co-authored-by: Greg PRice <greg@zulip.com>

zerver/lib/exceptions.py

@@ -52,6 +52,7 @@ class ErrorCode(Enum):
     REMOTE_BILLING_UNAUTHENTICATED_USER = auto()
     REMOTE_REALM_SERVER_MISMATCH_ERROR = auto()
     PUSH_NOTIFICATIONS_DISALLOWED = auto()
+    EXPECTATION_MISMATCH = auto()
 
 
 class JsonableError(Exception):
@@ -656,3 +657,15 @@ class TopicWildcardMentionNotAllowedError(JsonableError):
     @override
     def msg_format() -> str:
         return _("You do not have permission to use topic wildcard mentions in this topic.")
+
+
+class PreviousSettingValueMismatchedError(JsonableError):
+    code: ErrorCode = ErrorCode.EXPECTATION_MISMATCH
+
+    def __init__(self) -> None:
+        pass
+
+    @staticmethod
+    @override
+    def msg_format() -> str:
+        return _("'old' value does not match the expected value.")