Get rid of singleton SessionDataStorage

[aydenp]

Currently, the app is very reliant on SessionDataStorage. This is a huge fault as I try to move everything to a separate framework, BankKit, as it doesn't seem right for the framework to be housing this data in a singleton. Rather, the manager should find a way to create individual sessions, which can store a Plaid item container, which will perform similarly to how SessionDataStorage does right now, except tied to a given Item. This would require a substantial rewrite.

Eventually, when we want to add multiple bank logins per app, it'll also come in handy, as it doesn't make sense to have one for all.