Add balance tracking for individual users

Hi Zsolt,

Here's a pull request. The feature is not complete yet; The balance itself is not visible for any users, as I couldn't find an acceptable location / style to display it.
In addition, I'm not sure what the middle numbers of the .sql file should be, so I arbitrarily added "1234".
Lastly, I'm wasn't sure how to use a link to point to an action that processes it, so I created a small form with a single button instead that calls the recalculation.

As for the recalculation, it is done when a new record is added (for each of the users affected), and it looks through all the records that the user is attached to (only on active projects). This then gets stored in the "balance" column of the user itself.

Cheers,
Dani