Add persistence

[MartinRosenberg]

As is, messages from outside (before or after) the current session are unavailable. Storing the history and bringing it back on startup resolves this.

Suggested implementation

• Most relevant tutorials/documentation revolve around the MEAN stack, so MongoDB would be very quick/easy to add, and would work just fine for the sake of this app. Also, as object structure gets more complex, a JSON(-like) document schema gets easier compared to a relational DB.
• Right now, the data is pretty simple and small, so something simple like SQLite would be a great start. That would also ease the transition to a different SQL database coughpostgrescough if that's called for later.

[MartinRosenberg]

Firebase would be a good choice for a document store as well. It's cheap (or free), and I've used it before. Plus, they have a new thing out (Cloud Firestore, presumably a shop in the heavens where you can buy flames) that's supposed to be pretty nice (although I haven't used that one before).