Fix m2m unsaved at music creation
Imports in name forms
Minor change in base template

Added condition to display opus type help message
Model positive small integer optimization
Split detail_edit and start single mutualization

Split detail_edit into detail and edit for artists, opuses and musics
Mutualization of detail and edit functions for artists, opuses and
musics
Add main_name and other_names shorthands directly in Artist, Opus and
Music classes