Abstract all configuration data into a singleton called `configManager`, who is responsible for reading and writing to the persisted storage (localStorage, in our case).
Uses my new module [pojo-migrator](https://www.npmjs.com/package/pojo-migrator), and wraps it with the `ConfigManager` class, which we can hang any state setting or getting methods we need.
By keeping all the persisted state in one place, we can stabilize its outward-facing API, making the interactions increasingly atomic, which will allow us to add features that require restructuring the persisted data in the long term without having to rewrite UI or even `background.js` code.
All the restructuring and data-type management is kept in one neat little place.
This should make it very easy to add new configuration options like user-configured providers, per-domain vaults, and more!
I know this doesn't seem like a big user-facing feature, but we have a big laundry list of features that I think this will really help streamline.
In order to persist the seed word page until the user clicks the confirmation button, we need to store the seed words in localStorage.
To simplify this process I've also reorganized some of the account manager code, broken up one large function into many smaller functions, and created a new class for the IdMgmt object.
Again, sorry such a big refactor in one commit, but I really had to break it down to work through it.
