Use emacs web-mode as it supports DTL.

.editorconfig

@@ -11,6 +11,6 @@ indent_style = space
 indent_size = 4
 
 [*.html]
-file_type_emacs = jinja2
+file_type_emacs = web
 indent_style = space
 indent_size = 2

biscuit/core/templates/bootstrap4-navbar-submenu.html

+{# -*- engine:django -*- #}
 <div class="dropdown-menu dropdown-menu-right" aria-labelledby="navbarDropdown">
     {% for item in items %}
         {% if item.visible %}

biscuit/core/templates/bootstrap4-navbar.html

+{# -*- engine:django -*- #}
 {% for item in menu %}
     {% if item.visible %}
         {% if item.separator %}

biscuit/core/templates/core/base.html

+{# -*- engine:django -*- #}
 {% extends "bootstrap4/bootstrap4.html" %}
 {% load bootstrap4 menu staticfiles font_awesome any_js %}
 

biscuit/core/templates/core/index.html

+{# -*- engine:django -*- #}
 {% extends "core/base.html" %}
 {% load bootstrap4 %}
 

biscuit/core/templates/registration/login.html

+{# -*- engine:django -*- #}
 {% extends "core/base.html" %}
 {% load bootstrap4 %}