add users editable support

[``* `#17014` users editable``](http://community.openproject.org/work_packages/17014)
@0xF013 this definitely goes into the right direction. 

With the mentioned disclaimer "not my field of expertise and maybe just RoR biased": reading the words "render" and "data" in one sentence always strikes me as weird. If there is a data object it should only be responsible for handling that data. Another entity should know how to render the data (now I have done it myself).