ledger_lines in Invoice element is kludgy

We should move the ledger_lines from dicts() to proper objects. Right now this is a kludge:

see 698d0233d4398a14f79064b2c519995b393263a3
and see bosso-commit:
```
commit 0437faaebffe56eba7f2fa065f03e10951bbfb20
Date:   Tue Sep 4 14:23:24 2018 +0200

    billing::journal: Dirty hacks to get VAT push to exact to work
```