17228 spent time is not part of wp in api v3

[``* `#17228` Spent time not part of API V3's work package response``](https://community.openproject.org/work_packages/17228)

:exclamation: Remove ``model`` when merging this into dev branch.
@hschink do we need to handle pluralisation? I think it'd be better to specify something here that the front-end understands and, if necessary, can reformat (either i18n.js or Moment.js should be able to help us).