update api and webui:

1. add download_doc to api
2. return local path or http url in kowledge_base_chat depends on
   no_remote_api
3. change assistant avater in webui

server/chat/knowledge_base_chat.py

@@ -1,4 +1,4 @@
-from fastapi import Body
+from fastapi import Body, Request
 from fastapi.responses import StreamingResponse
 from configs.model_config import (llm_model_dict, LLM_MODEL, PROMPT_TEMPLATE,
                                   VECTOR_SEARCH_TOP_K)