update matrix_bot_chatgpt_version to 3.1.0

It wasn't used at all. It doesn't seem like there's a way to control
the log level, judging by:
- 8f9eea4114/.env.example
- 8f9eea4114/src/env.ts

OpenAI changed the model a month ago and broke the bot.
This bump fixes it and adds the default model.

Use matrix_bot_chatgpt_context for CHATGPT_CONTEXT

matrix_bot_chatgpt_context=thread

Set OPEN_AI to matrix_bot_chatgpt_openai_pro

Add support for OPENAI_PRO for ChatGPT pro subscriptions.

This container needs a writable $HOME, and will fail at startup if
there isn't one.

Provide one by pointing HOME to a path under the mounted /data
directory.

Co-Authored-By: Slavi Pantaleev <slavi@devture.com>