browser (template): adapted filters/global/extensions to manage SàT templates: `template` module has been update so most SàT template can be run from browser: - `profile` and `csrf_token` are set as globals - an implementation of `xmlattr` filter has been added - `date_fmt` filter has been implemented using `moment.js` - i18n method `_` has been added to globals, and `{% trans %}` statement has been implemented using an extension. For now they are not actually translating but just returning the unmodified string. - new `get_args` helper method to handle `nunjucks` convention for arguments. - fixed `get_elt` to only return the first child element (avoiding any text child) + added a defaut value for `context`