add new mailer and fix old ones

app/mailers/notification_mailer.rb

app/models/import.rb

app/models/user.rb

app/models/user/with_registration_context.rb

app/views/admin/imports/show.html.erb

app/views/mailers/notifications/low_sms_credits.html.erb

app/views/mailers/notifications/new_registration.html.erb

...ilers/notifications/website_invalid_access_token.html.erb

config/locales/en.yml

config/locales/fr.yml

db/migrate/20231013090313_change_imports_user_nil.rb

db/schema.rb

test/mailers/previews/notification_mailer_preview.rb