Move the welcome modal to an interstitial, add import button (#762)

I noticed that the first run flow is suboptimal for people who want to
import an existing file from Actual/YNAB. I’ve moved the welcome modal
into the management app and set it up to appear when there are no
budgets available (which also has the benefit of allowing people to see
the modal again!)

I think there’s some weirdness around getting the modal to reappear when
deleting a budget file which I want to work out before merging this.

This PR also reorganizes the management app a bit to reduce usage of
modals (currently, hitting escape while the budget list is open leaves
you with a blank page).

<img width="539" alt="Screenshot_2023-03-18 08 53 54"
src="https://user-images.githubusercontent.com/25517624/226107462-b2b88791-1015-4397-b290-c64e7fcc0f41.png">

- [x] Ensure modal consistently appears when needed (no longer a modal!)
- [x] Fix e2e tests