Use shared chat-instruct_command with api

[jordanbtucker]

Checklist:

• I have read the Contributing guidelines.

This PR sets chat-instruct_command to the shared UI setting when called from the API, making the corresponding chat_instruct_command API parameter optional when using chat-instruct mode.

Without this PR, when calling the API with chat-instruct mode, the following error is thrown when no chat_instruct_command parameter is provided.

  File "text-generation-webui\modules\chat.py", line 93, in generate_chat_prompt
    command = state['chat-instruct_command'].replace('<|character|>', state['name2'] if not impersonate else state['name1'])
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'replace'

With this PR, the API call succeeds when the chat_instruct_command parameter is not provided since it defaults to the UI setting.