[Developers] Allowing imdb keys to be asserted on more types

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-567

Sorry, this bug has been in the queue for so long that I forgot to  
send out a note out about it before doing the actual refactoring;  
however, there shouldn't be any app breakage.

These three properties have been moved:

/film/film_character/imdb_profile
/film/actor/imdb_entry
/film/film/imdb_id

Previously, the imdb name namespace was linked to the /film/actor/ 
imdb_entry.  This was problematic because, for example, not everyone  
in the imdb name namespace is an actor.  This refactoring allows us to  
assert imdb keys on a wider scope of topics.

They will no longer show up in the schema browser, but because they  
still have their respective keys, queries relying on those properties  
won't break.

Bryan