Add authentication from upstream providers (#107)

Let BasicOIDC delegate authentication to upstream providers (Google, GitHub, GitLab, Keycloak...)

Reviewed-on: #107

templates/login/prov_login_error.html

@@ -0,0 +1,13 @@
+{% extends "base_login_page.html" %}
+{% block content %}
+
+<div class="alert alert-danger" style="margin-bottom: 10px;">
+    <strong>Authentication failed!</strong>
+
+    <p style="margin-top: 10px; text-align: justify;">{{ message }}</p>
+</div>
+
+<a href="/login?redirect={{ _p.redirect_uri.get_encoded() }}">Go back to login</a>
+
+
+{% endblock content %}