m2m-json: fixed json_to_list so that it doesn't break other scripts if json field is empty
m2m-json: moved forms.js to forms_m2m_json.js
Forgot to delete original
Added Creation abstract model as a base class for all creation object models (Poetry, Music, Recording...)
get_parent and get_title methods allow various object-agnostic processing, including output
Changed fields order in the creation models to comply with "base-derived" logic
Common code for filling forms context dict moved to utils.py
Added links to create derived objects under creation forms, which point to referencing objects creation form pages with matching fk fields pre-filled with current object id
Added object_info template which, together with model get_parent and template properties, allows model-agnostic object rendering
Implemented title/related-piece input, which follows our "base-derived" creation objects relationship logic: user can use title input to either finde&choose base object for the one being edited (e. g. music for recording, poetry for music) if the object should have one, or enter title string, if it's standalone