Correctly map transfers between asset accounts that are not imported by Luciferin

[bramhoven]

In Firefly III you can specify many asset accounts. Some of these might not be able to be imported by Luciferin. For example a savings account with SNS cannot be connected through Nordigen.

We should be able to detect transactions from or to those accounts from the accounts we are able to import. We can then directly import these as transfers instead of having to move them with rules within Firefly III. This will take a require a good look at the current transaction creation and import process, and it will probably have to be restructured.

Somthing along these lines:
Nordigen -> Mapping -> Duplicate detection -> Transfer detection -> Importing