[migrations] Initialize real balance to initial balance + sum of transactions...

Worked fine on imports for me but I can't seem to test in on by switching from a previous commit (4888bf6a) to this last commit, it hangs right after logging the SQL: https://framagit.org/nicofrand/kresus/-/blob/master/server/models/entities/accounts.ts#L204.

I don't know if this comes from this branch or my env.

@bnjbvr Could you test it please? With postgresql (since sqlite will delete transactions when altering the account table).