OFS-118: fixing replace mutations and replace in model

[sniedzielski]

In that pull request I want to merge some changes related to "replace" mutation and changes connected with this operation.

I also changed the replace mutation in base mutations.

Furthermore I had to make changes in Core module in model so as to allow update fields if somebody want to change certain fields.

Another pull requests related to this issue:

• policy holder: OFS-118: fixing replace mutation in PolicyHolder openimis-be-policyholder_py#12
• contribution plan: OFS-118: fixing 'replace' mutation in CP module openimis-be-contribution_plan_py#9

[sniedzielski]

Please nor merge until I create another pull requests.