fix: timeout on spa manifest requests

The previous behaviour had a loop of requests between the front
app and the api when querying the pwa manifest.

This reduce the coupling around the pwa manifest file between the api
and the front app, by uplicating the files so each "service" has a copy
of it, while keeping them in sync and having the front pwa manifest as
single source of truth.

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2291>

api/tests/instance/test_views.py

@@ -1,4 +1,4 @@
-import json
+from unittest import mock
 
 from django.urls import reverse
 
@@ -38,20 +38,21 @@ def test_admin_settings_correct_permission(db, logged_in_api_client, preferences
     assert len(response.data) == len(preferences.all())
 
 
-def test_manifest_endpoint(api_client, mocker, preferences, tmp_path, settings):
-    settings.FUNKWHALE_SPA_HTML_ROOT = str(tmp_path / "index.html")
-    preferences["instance__name"] = "Test pod"
-    preferences["instance__short_description"] = "Test description"
-    base_payload = {}
-    manifest = tmp_path / "manifest.json"
-    expected = {
-        "name": "Test pod",
-        "short_name": "Test pod",
-        "description": "Test description",
-    }
-    manifest.write_bytes(json.dumps(base_payload).encode())
+def test_manifest_endpoint(api_client, preferences):
+    with mock.patch(
+        "funkwhale_api.instance.views.PWA_MANIFEST",
+        {"lang": "unchanged"},
+    ):
+        preferences["instance__name"] = "Test pod"
+        preferences["instance__short_description"] = "Test description"
+        expected = {
+            "lang": "unchanged",
+            "name": "Test pod",
+            "short_name": "Test pod",
+            "description": "Test description",
+        }
 
-    url = reverse("api:v1:instance:spa-manifest")
-    response = api_client.get(url)
-    assert response.status_code == 200
-    assert response.data == expected
+        url = reverse("api:v1:instance:spa-manifest")
+        response = api_client.get(url)
+        assert response.status_code == 200
+        assert response.data == expected