add interchangable model keys support

[PetrDlouhy]

Add possibility to change target object key field by COMMENTS_ID_OVERRIDES settings, e.g.:

COMMENTS_ID_OVERRIDES = {
    'assets.Asset': 'asset_uuid',
}

I have model with id field as PK and want to use another non-PK uuid field to use for comments (to avoid showing object IDs to users).

This is conceptual draft of the functionality that enables that through one simple mapping in settings.

Please review if this concept makes sens. If yes, I will do following things:

• Check, that the new settings is used everywhere.
• Add documentation
• Add tests

[PetrDlouhy]

Nobody has commented this, so I decided to finish the task. I have added documentation and test routines.

Now I think this is ready to be pulled.

[blockchainGuru1018]

it's looks good in my view @PetrDlouhy

[PetrDlouhy]

@felixxm Can you please take a look at this PR?

[PetrDlouhy]

@claudep @felixxm Could you please tak a look at this PR?