Transaction Import: Duplicate external accounts created

[morremeyer]

Describe the bug
When importing to the same external account during one import, it is created as many times at is is referenced by a transaction instead of only once.

Root cause
When an account is created during the import, it is loaded from the backend to be available for other transactions, but not automatically selected for transactions with the same *accountName.

[morremeyer]

This is now prevented by the backend erroring on duplicate account names, but the frontend should still transparently "re-load" the accounts once a new one is created