update DB to store individual positions instead of total value only

[MarkusZoppelt]

Currently, the db only stores the previous total prices and not the individual stocks.

Example:

Adding a new position (that has performed poorly this YTD) will change the historical performance of the portfolio, even though it's a new position.

Todo:

keep track of all individual stocks (that way buying and selling stocks would become new db entries). This would help tracking the performance better.

Thanks to u/fynnsu for bringing up that issue.