Make templates extending base template

aleksis/core/templates/templated_email/celery_failure.email

 {% load i18n %}
-
+{% extends "base.html" %}
 {% block subject %}
  {% blocktrans with task_name=task_name%} Celery task {{ task_name }} failed!{% endblocktrans %}
 {% endblock %}
 
 {% block plain %}
- {% trans "Hello," %}
  {% blocktrans with task_name=task_name %}
    the celery task {{ task_name }} failed with following information:
  {% endblocktrans %}
@@ -23,7 +22,6 @@
 {% endblock %}
 
 {% block html %}
- <p>{% trans "Hello," %}</p>
  <p>
   {% blocktrans with task_name=task_name %}
     the celery task {{ task_name }} failed with following information:

aleksis/core/templates/templated_email/data_checks.email

 {% load i18n %}
-
+{% extends "base.html" %}
 {% block subject %}
  {% trans "The system detected some new problems with your data." %}
 {% endblock %}
 
 {% block plain %}
- {% trans "Hello," %}
 
  {% blocktrans %}
   the system detected some new problems with your data.
@@ -18,10 +17,6 @@
 {% endblock %}
 
 {% block html %}
- <style>
-  {% include "templated_email/data_checks.css" %}
- </style>
- <p>{% trans "Hello," %}</p>
  <p>
   {% blocktrans %}
    the system detected some new problems with your data.

aleksis/core/templates/templated_email/notification.email

 {% load i18n %}
-
+{% extends "base.html" %}
 {% block subject %} {% trans "New notification for" %} {{ notification_user }} {% endblock %}
 
 {% block plain %}
-    {% blocktrans with notification_user=notification_user %}Dear {{ notification_user }},{% endblocktrans %}
+    {% blocktrans with notification_user=notification_user %} {{ notification_user }},{% endblocktrans %}
 
     {% trans "we got a new notification for you:" %}
 
@@ -23,8 +23,7 @@
 {% endblock %}
 
 {% block html %}
-<main>
-    <p>{% blocktrans with notification_user=notification_user %}Dear {{ notification_user }},{% endblocktrans %}</p>
+    <p>{% blocktrans with notification_user=notification_user %} {{ notification_user }},{% endblocktrans %}</p>
 
     <p>{% trans "we got a new notification for you:" %}</p>
 
@@ -42,8 +41,4 @@
         {% endblocktrans %}
     </p>
 
-    <p>
-        <i>{% trans "Your AlekSIS team" %}</i>
-    </p>
-</main>
 {% endblock %}

aleksis/core/templates/templated_email/person_changed.email

 {% load i18n %}
-
+{% extends "base.html" %}
 {% block subject %}
  {% blocktrans with person=person %}{{ person }} changed their data!{% endblocktrans %}
 {% endblock %}
 
 {% block plain %}
- {% trans "Hello," %}
 
  {% blocktrans with person=person %}
    the person {{ person }} recently changed the following fields:
@@ -17,7 +16,6 @@
 {% endblock %}
 
 {% block html %}
- <p>{% trans "Hello," %}</p>
  <p>
   {% blocktrans with person=person %}
     the person {{ person }} recently changed the following fields: