Add multiplier support to intervals

[snorremd]

Sometimes when tracking time you may want that time to count extra. You might for example be working on a weekend and want that time to count 150%. This could be solved by adding a multiplier value to each interval, defaulting to 1.

The intervals should still track the actual timestamps and time worked, and the API will still return these values unchanged. But when calculating stats we can use the multiplier to adjust the worked time. This will affect the statistics on the dashboard as well as the stats on the statistics page.