Support Templates in SMTP Extension

[derks]

Moving this bit from PR #742:

def send_by_template(self, template, data={}, **kw):
        # test if template exists by loading it
        def _template_exists(template):
            # check if stored in cache already
            if template in self._template_exists_cache:
                return self._template_exists_cache[template]
            # do first time check from file or module
            result = False
            try:
                # successfully load when available
                self.app.template.load(template)
                result = True
            except:
                pass
            # store flag in cache list to prevent often load access
            self._template_exists_cache[template] = result
            # return state
            return result

        # prepare email params
        params = dict(**kw)
        # check render subject
        if 'subject' not in params:
            if _template_exists(f'{template}.title.jinja2'):
                params['subject'] = self.app.render(data,
                                                    f'{template}.title.jinja2',
                                                    out=None)
        # build body
        body = list()
        if _template_exists(f'{template}.plain.jinja2'):
            body.append(self.app.render(dict(**data, mail_params=params),
                                       f'{template}.plain.jinja2',
                                       out=None))
        if _template_exists(f'{template}.html.jinja2'):
            # before adding a html part make sure that plain part exists
            if len(body) == 0:
                body.append('Content is delivered as HTML only.')
            body.append(self.app.render(dict(**data, mail_params=params),
                                        f'{template}.html.jinja2',
                                        out=None))
        # send the message
        self.send(body=body, **params)

Would be convenient to be able to link templates with sending mail, however it needs to be TemplateHandler agnostic.