Overhaul how NeoChat restores rooms on space switch

I'm pretty unhappy with the current state, almost no other chat application (including other Matrix clients) work like this. The biggest annoyance is opening other spaces will send you to the Space Home - which is almost never what you want! Even though our Space Home is awesome :-)

Now when you switch spaces, the last room in that space is stored and will be restored. This also allows us simplify some of the "open the last room" code. I made sure that this works for the two special spaces "Home" and "Friends" so everyone can take advantage of this better UX.

Since this is constantly-changing-state and the worst that can happen is the wrong room opening on launch, I didn't bother migrating the existing setting. We'll have to kill it eventually anyway when we make it account-specific!