add users editable support

[``* `#17014` users editable``](http://community.openproject.org/work_packages/17014)
This is absolutely not my field of expertise but it seems to me that the structure of this code is the wrong way around. What is done here is the overview.html knowing every possible value for which it needs to handle the attribute in a special way. 
* Extending this to include other fields requires the file to change. 
* Changing the behaviour of one field requires the file to change. 
* I am not sure how we would handle plugins this way. 

This IMO violates the SRP. Additionally, this will result in a very long case statement.

Each attribute should have a means to be asked how to be rendered. A decorator comes to mind but that might be just my RoR way of thinking.   

@0xF013, @hschink, @myabc opinions?