CPS should not allow trailing white spaces on entity codes

[takavarasha]

Currently, CPS allows trailing white spaces to per stored on entity codes. Assuming codes are meant to be unique, allowing trailing white spaces gives rise to the possibility of duplicate entities being created. Please see the two ZWE on the Curated Data/Entities page which differ in that the other has a trailing space.

[seustachi]

There was indeed two ZWE entities, but I could not reproduce the bug. I added some unit tests to make sure this cannot happen.