Move to a REST-ier URI API.

Account keys are moved from the query string to the path.

apiUrl is made more flexible to allow arbitrary numbers of elements.
Align amounts on the right.
Simplify returning to account list with refresh.

Since we'll often want to start updating the account list then move to
that page, extract it to a separate method.
Implementation of DeleteAccount.

If an account has splits, it's not deletable, but otherwise it can be
removed. Since there's no way to remove splits right now, this only
allows you to delete accounts created but never used for a transaction.
Add account deletion to the frontend.