Create functionalty for extension points

[markoIvetic93]

No description provided.

[markoIvetic93]

Short description of usage (and implementation):

• Generated code calls functions, which are in another file (customs.py). Those functions are empty first, and user can put custom code there, and that file will not be overridden.

[sergiosuperstar]

Proposed changes for Django.Core:

1. Create folder for custom functions. Proposed folder name: "model_extensions"
2. Create custom functions file per model jinja template. Proposed generated file name: "{{ model_m }}_extensions.py"
3. Create jinja template in "model_extensions" folder.
4. {{model_m}} file content: python function or functions for model

Proposed changes for Generator:

1. Update how createCustomFiles work by creating {{ model_m}}_extensions.py per model
2. Move if file exist check from execute.py into createCustomFiles.py
3. Update if file exit check so that it checks if file exist per model