Support setting per-project hours targets

When planning a sprint, our goals always define a rough allocation of hours per project. The project manager user ought to be able to add those goals to a project, and the interface ought to display them and warn you when the total allocation is close or over.

No files, tests, or entry points are named in the issue, so first locate the project model and sprint-planning interface. Done means project managers can set per-project hour targets, the interface displays them, and it warns when the total allocation is close to or above the target.

Written by the indexing model from the issue text.