Re-use chains

# Overview

Instead of creating a new chain with each prompt, we should create the chain once (maybe when the chatify extension is first loaded?) and re-use it to respond to each query.

# Completion criteria

This requires refactoring [this function](https://github.com/ContextLab/chatify/blob/200e0a6dd1982ad241a59325db458aac3f0467d6/chatify/main.py#L141) and [this function](https://github.com/ContextLab/chatify/blob/200e0a6dd1982ad241a59325db458aac3f0467d6/chatify/main.py#L201)
- [ ] create a new chain when the extension is first loaded
- [ ] execute the chain when new queries are submitted

# Resources (optional)

Mentioned [here](https://github.com/ContextLab/chatify/blob/200e0a6dd1982ad241a59325db458aac3f0467d6/chatify/main.py#L156).