🧑‍💻(views) render email's template

THis feature is inspired by Joanie. Add two new urls to render Emails
HTML and Text templates.

Developpers can render the email template they are working on. When necessary,
run make mails-build, and reload `_debug__/mail/hello_html`, it will re-render
the updated email template.

Also, I have copy/pasted one template extra tags from Joanie, which loads
bas64 string from static images. This code is necessary to render the dummy
template `hello.html`.

src/backend/core/templatetags/extra_tags.py

@@ -0,0 +1,58 @@
+"""Custom template tags for the core application of People."""
+
+import base64
+
+from django import template
+from django.contrib.staticfiles import finders
+
+from PIL import ImageFile as PillowImageFile
+
+register = template.Library()
+
+
+def image_to_base64(file_or_path, close=False):
+    """
+    Return the src string of the base64 encoding of an image represented by its path
+    or file opened or not.
+
+    Inspired by Django's "get_image_dimensions"
+    """
+    pil_parser = PillowImageFile.Parser()
+    if hasattr(file_or_path, "read"):
+        file = file_or_path
+        if file.closed and hasattr(file, "open"):
+            file_or_path.open()
+        file_pos = file.tell()
+        file.seek(0)
+    else:
+        try:
+            # pylint: disable=consider-using-with
+            file = open(file_or_path, "rb")
+        except OSError:
+            return ""
+        close = True
+
+    try:
+        image_data = file.read()
+        if not image_data:
+            return ""
+        pil_parser.feed(image_data)
+        if pil_parser.image:
+            mime_type = pil_parser.image.get_format_mimetype()
+            encoded_string = base64.b64encode(image_data)
+            return f"data:{mime_type:s};base64, {encoded_string.decode('utf-8'):s}"
+        return ""
+    finally:
+        if close:
+            file.close()
+        else:
+            file.seek(file_pos)
+
+
+@register.simple_tag
+def base64_static(path):
+    """Return a static file into a base64."""
+    full_path = finders.find(path)
+    if full_path:
+        return image_to_base64(full_path, True)
+    return ""