Resolve "2FA"

Closes #91 (closed) TODO:

• https://django-two-factor-auth.readthedocs.io/en/stable/installation.html#yubikey-setup
• Add custom settings for two_factor (Gateways, max digits for codes,…)
• Rebuild templates
• Add two_factor settings to account menu

biscuit/core/templates/two_factor/login.html

+{# -*- engine:django -*- #}
+{% extends "core/base.html" %}
+{% load bootstrap4 i18n %}
+
+{% block content %}
+ {% if next %}
+  {% if user.is_authenticated %}
+   <p>
+    Your account doesn't have access to this page. To proceed,
+    please login with an account that has access.
+   </p>
+  {% else %}
+   <p>Please login to see this page.</p>
+  {% endif %}
+ {% endif %}
+
+ <form method="post" action="{% url 'two_factor:login' %}">
+  {% csrf_token %}
+  {% bootstrap_form form %}
+  <button type="submit" class="btn btn-dark">
+    {% blocktrans %}Login{% endblocktrans %}
+  </button>
+ </form>
+{% endblock %}