Add 'get_template_extension(typename)' method

Add

- `activities.mediator.ActivityMediator.default_template_extension`
- `activities.mediator.ActivityMediator.template_extensions`
- `activities.mediator.ActivityMediator.get_template_extension(typename)`
- `django.conf.settings.ACTIVITIES_DEFAULT_TEMPLATE_EXTENSION`
- `django.conf.settings.ACTIVITIES_TEMPLATE_EXTENSIONS`

to control template extension by `typename`.