Support multiple chat threads

[chrisglein]

Summary

Likely would build on #104, otherwise the multiple chat threads wouldn't exist past a single session.

User interface for this is a big part of it. On the ChatGPT website there is something like a navigation view on the left, with multiple chat threads (summarized #31) as individual list items. We could bring in react-navigation for this (as RN Gallery does), or figure out something else. But the fork is likely going to be at the ChatSession level, with App remaining unchanged.

With multiple threads, the 💣 starts to make less sense (assuming each chat session can be ❌ from the list), unless combined with #65.