These will be used to show the amount logged against specific time
budgets on each project. It’s potentially a little naïve but we can
start with the simpler approach and see.
Refs #2165
Add a TimeBudget model which allows a job to have a time budget for each
activity type. We’re allowing nil as a holder for a generic budget,
though there’s currently no way to use that.
We automatically delete any time budgets which are set to zero.