Multi-user project dedication report

[jaragunde]

This report would show the time devoted by some users to every project during a period of time. Users would be listed in rows, and projects in columns:

|           | PhpReport development | Server maintenance |
| jaragunde |           100         |        0           |
| pabelenda |            35         |        300         |

The report would provide the following filters:

• Date filters, as usual in most reports.
• A multi-selection field for users, where you can manually write who will be listed (could this widget be used to implement it?).
• A project selector. It would populate the multi-selection field with project members.
• An area selector. It would populate the multi-selection field with the users who are members at that moment.

The date filters are currently implemented with the DateIntervalForm widget, which might be reused. The user filters defined should ideally be implemented in a way they can be reused, for example, to implement #430.

Finally, this new report would replace both "users evaluation" and "projects summary" reports. Check if part of their backend operations can be reused to implement this report.

Originally from #331.