16972 add assignee responsible to wp form

[``* `#16972` Support assignee/responsible property on work package forms and PATCH endpoint``](https://community.openproject.org/work_packages/16972)

Includes https://community.openproject.org/work_packages/17426

- [ ] Path service
Do we have access to the AR (and it's connection to I18n) here? Just asking because we probably shouldn't have to many human readable strings within the code. 

We can definitely do someting like ```I18n.t('activerecord.errors.messages.inclusion', locale: :en)``` although it might make sense to take another look at what reform does with regards to errors and their localisation.

The ```inclusion``` might not offer the best translation, though. I just propose moving stuff like this to the localisation files.

The message could be something like "is not within the list of valid users".