Don't fail Python tests for missing SVG icons

We don't want to have to run the frontend build to run Python tests.

settings.py

@@ -51,13 +51,22 @@
     {
         "BACKEND": "django.template.backends.django.DjangoTemplates",
         "DIRS": [BASE_DIR / "viewer/static/icons"],
-        "APP_DIRS": True,
         "OPTIONS": {
             "context_processors": [
                 "django.template.context_processors.debug",
                 "django.template.context_processors.request",
                 "viewer.context_processors.crawl_stats",
             ],
+            "loaders": [
+                (
+                    "django.template.loaders.cached.Loader",
+                    [
+                        "django.template.loaders.filesystem.Loader",
+                        "django.template.loaders.app_directories.Loader",
+                        "viewer.loader.IgnoreMissingSVGsTemplateLoader",
+                    ],
+                ),
+            ],
         },
     },
 ]

viewer/loader.py

@@ -0,0 +1,16 @@
+import re
+
+from django.template.base import Origin
+from django.template.loaders.base import Loader
+
+
+SVG_FILENAME = re.compile(r"^.*\.svg$")
+
+
+class IgnoreMissingSVGsTemplateLoader(Loader):
+    def get_template_sources(self, template_name):
+        if SVG_FILENAME.match(template_name):  # pragma: no branch
+            yield Origin(template_name, template_name, loader=self)
+
+    def get_contents(self, origin):
+        return ""