Activities management for Responsables

+ update of forms for depecrated fields (TextField, Required)
Missing: data upload
Role-specific menus
Flashes & messages central integration
Assets management for activities

- upload module allows adding, listing, deleting files
- responsable accesses those features
- assets retrieval for any user
Activite pages for any user
Acitivite: model, form and view updates

- "nombre_volontaires" field in the model
- form: correct handling of None datetimes at the beginning
- template: assets upload not available on activite creation
- activite folder is its id and not its manager email
- login: fixed message on empty parameter