Improve and simplify Khoj Chat using ChatGPT

- Set context by either including last 2 chat messages from active
  session or past 2 conversation summaries from conversation logs

- Set personality in system message
- Place personality system message before last completed back & forth
  This may stop ChatGPT forgetting its personality as conversation progresses given:
  - The conditioning based on system role messages is light
  - If system message is too far back in conversation history, the
    model may forget its personality conditioning
  - If system message at end of conversation, the model can think its
    the start of a new conversation
  - Inserting the system message before last completed back & forth should
    prevent ChatGPT from assuming its the start of a new conversation
    while not losing personality conditioning from the system message

- Simplfy the Khoj Chat API to for now just answer from users notes
  instead of trying to infer other potential interaction types.
  - This is the default expected behavior from the feature anyway
  - Use the compiled text of the top 2 search results for context

- Benefits of using ChatGPT
  - Better model
  - 1/10th the price
  - No hand rolled prompt required to make GPT provide more chatty,
    assistant type responses

pyproject.toml

@@ -40,7 +40,7 @@ dependencies = [
     "defusedxml == 0.7.1",
     "fastapi == 0.77.1",
     "jinja2 == 3.1.2",
-    "openai == 0.20.0",
+    "openai >= 0.27.0",
     "pillow == 9.3.0",
     "pydantic == 1.9.1",
     "pyqt6 == 6.3.1",