Refactor initial prices sync

[chedieck]

Currently it works in a weird way: loading a comitted file that is not updated automatically and is periodically updated by having someone deleting it, thus having it be regenerated up-to-date and then commiting it in an unrelated PR.

It should:

1. Check a git ignored file for prices.
2. If the file has prices, make sure that they are all consecutive (no gaps)
3. Then compare the oldest price date with today and fetch the lacking prices
4. Put those prices at the end of the file, so it is up-to-date for the next run.