add users editable support

[``* `#17014` users editable``](http://community.openproject.org/work_packages/17014)
There is a misunderstanding of the requirements here. As opposed to the way "Description" is handled, the placeholder for assignee/responsible not being set is simply a "-".

This might not be ideal as long as the attribute is not styled, but once it is every row will span the complete width of the details pane. At least, the edit button will be placed at the end of each line and there will be a border around it.

Please see http://design.prototype.openproject.org/#p=details_pane_-_inplace_edit_-_version_3 for reference