add users editable support

[``* `#17014` users editable``](http://community.openproject.org/work_packages/17014)
@ulferts I think the 'data' here is just a name issue of the variable. The question is: should the rendering strategy be set on the server and returned in an API response or it is the responsibility of the front-end? I guess we don't need that level of customization at the moment so I propose to add a front-end service that accepts a propertyData object and returns the properties that are fed to the inplace-editor directive (I guess that's a proxy)