Intervention model refactoring

* adds UserActionLogEntry model to user/models.py
  * wraps user and timestamp info
  * can be extended for more information in the future
* refactors all filtering and accessing on values

konova/enums.py

@@ -47,3 +47,11 @@ class ServerMessageImportance(BaseEnum):
     DEFAULT = "DEFAULT"
     INFO = "INFO"
     WARNING = "WARNING"
+
+
+class UserActionLogEntryEnum(BaseEnum):
+    """
+    Defines different possible user actions for UserActionLogEntry
+    """
+    CHECKED = "Checked"
+    RECORDED = "Recorded"