[utils] Add email sending helper

Most email sending code follows the same logic, these helpers
allow to avoid all the code duplication.

beat/web/utils/mail.py

+#!/usr/bin/env python
+# vim: set fileencoding=utf-8 :
+
+###############################################################################
+#                                                                             #
+# Copyright (c) 2018 Idiap Research Institute, http://www.idiap.ch/           #
+# Contact: beat.support@idiap.ch                                              #
+#                                                                             #
+# This file is part of the beat.web module of the BEAT platform.              #
+#                                                                             #
+# Commercial License Usage                                                    #
+# Licensees holding valid commercial BEAT licenses may use this file in       #
+# accordance with the terms contained in a written agreement between you      #
+# and Idiap. For further information contact tto@idiap.ch                     #
+#                                                                             #
+# Alternatively, this file may be used under the terms of the GNU Affero      #
+# Public License version 3 as published by the Free Software and appearing    #
+# in the file LICENSE.AGPL included in the packaging of this file.            #
+# The BEAT platform is distributed in the hope that it will be useful, but    #
+# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY  #
+# or FITNESS FOR A PARTICULAR PURPOSE.                                        #
+#                                                                             #
+# You should have received a copy of the GNU Affero Public License along      #
+# with the BEAT platform. If not, see http://www.gnu.org/licenses/.           #
+#                                                                             #
+###############################################################################
+
+"""
+Email related helper
+"""
+
+from django.conf import settings
+from django.core.mail import send_mail
+from django.template import loader
+
+
+def send_email(
+    subject_template,
+    message_template,
+    context,
+    recipients,
+    sender=settings.DEFAULT_FROM_EMAIL,
+):
+    """
+    Send email based on the given templates and context information to the list
+    of recipients using the sender address
+
+    Parameters:
+        subject_template (str): Path to subject template
+        message_template (str): Path the message template
+        context (dict): context data for the renderer
+        recipients (list): email address list of the email receivers
+        sender (str): email address of the sender
+    """
+
+    assert type(recipients) is list
+    subject, message = build_elements(subject_template, message_template, context)
+    send_mail(subject, message, sender, recipients, sender)
+
+
+def build_elements(subject_template, message_template, context):
+    """
+    Build email elements to send
+
+    Parameters:
+        subject_template (str): Path to subject template
+        message_template (str): Path the message template
+        context (dict): context data for the renderer
+    """
+
+    template = loader.get_template(subject_template)
+    subject = template.render(context)
+
+    # Note: e-mail subject *must not* contain newlines
+    subject = settings.EMAIL_SUBJECT_PREFIX + "".join(subject.splitlines())
+
+    template = loader.get_template(message_template)
+    message = template.render(context)
+
+    return (subject, message)