Rework account views and addition flow

[zachgoll]

The most common feedback that we have received about Maybe so far is around the addition of new accounts.

• “What does my current balance represent?”
• “I added a transaction, but my balance did not update”
• “I transferred money from one account to another, but the receiving account didn’t change value”

This PR aims to make the app more intuitive in the area of adding accounts.

Current behavior (confusing to users)

In the Maybe app, each account has a current balance. In the screenshot above, the “current balance” is $500.25

When you add, edit, or remove a transaction OR make a transfer to/from this account, you might expect that the balance should change. But it does NOT, and here’s why:

You can think of current balance as “bank balance” OR “current balance” or “today’s balance”

We ask for this balance when you create an account via CSV or manually:

If a user tells us, “my current balance is $5,000”, then no matter what transactions are added to the account, we assume that the current balance should remain $5,000.

Our goal was to keep this balance in sync with your actual current bank balance.

Adding/editing/removing transactions will alter the historical balances on the account, but not the “current” because we use the "bank balance" the user provided to "work backwards" to generate the historical balances based on transactions.