[DEV-1152-2] Add AskbotUserRelatedModel class.
[DEV-1152-2] Add Meta class to AskbotUserRelatedModel.
[DEV-1152-2] Add _prefix_user_fields method to DRY out processing of ORM query string.
[DEV-1152-2] Update _preprocess_Q_and_kwargs to use _prefix_user_fields method.
[DEV-1152-2] Update _preprocess_args to use _prefix_user_fields method.
[DEV-1152-2] Update _preprocess_field method to use _prefix_user_fields method. Fix _preprocess_field's call to its superclass method (was passing wrong arguments).
[DEV-1152-2] Update _preprocess_args with call to map, rather than manually building new args list.
[DEV-1152-2] Update _preprocess_Q_and_kwargs with unpacked dictionary comprehension.
[DEV-1152-2] Updates across preprocessor methods for style consistency.
[DEV-1152-2] Update dictionary comprehension in _preprocess_Q_and_kwargs with items().
[DEV-1152-2] Update _traverse_Q to use _prefix_user_fields method.
[DEV-1152-2] Minor style, comment & doc tweaks.
[DEV-1152-2] Fix import of AskbotUser from askbot.models.askbot_user to askbot.models.
[DEV-1152-2] Add except for FieldDoesNotExist to attempt getting model in two ways.
[DEV-1152-2] Remove special treatment of pk field in favor of excepting all FieldDoesNotExist errors in one place.
[DEV-1152-2] Remove AskbotUserRelatedModel abstract model: it's not going to work out.
[DEV-1152-2] Add PassThrough functionality to PostRevisionManager. Attach AskbotUserQuerySet to PostRevision via new manager.
[DEV-1152-2] Add declaration of AskbotUserQuerySet to models __init__.
[DEV-1152-2] Inherit from PassThroughManager rather than using the mixin class.
[DEV-1152-2] Add use_for_related_fields flag to PostRevisionManager.