Transaction inserting: Support adding per-posting flags

[pmarciniak]

I think the canonical example here is transfer between 2 bank accounts. The 2 bank accounts will be reconciled at different dates, so instead of marking the whole transaction with "!" flag (i.e. to-reconcile), I'd like to mark both postings with "!" flags and remove them one by one as I reconcile the 2 bank accounts.

Currently this is not supported by fava's transaction adding dialog.

[yagebu]

Like with #1147, I think this would overcrowd the form.