Make custom pages compatible with base_url setting

Closes #1238

- base_url no longer causes custom page routing to fail
- new route_path key in request.scope storing the path that was used for routing with the base_url prefix stripped
- TestClient used by tests now avoids accidentally double processing of the base_url prefix

tests/test_custom_pages.py

@@ -2,11 +2,19 @@ import pathlib
 import pytest
 from .fixtures import make_app_client
 
+TEST_TEMPLATE_DIRS = str(pathlib.Path(__file__).parent / "test_templates")
+
 
 @pytest.fixture(scope="session")
 def custom_pages_client():
+    with make_app_client(template_dir=TEST_TEMPLATE_DIRS) as client:
+        yield client
+
+
+@pytest.fixture(scope="session")
+def custom_pages_client_with_base_url():
     with make_app_client(
-        template_dir=str(pathlib.Path(__file__).parent / "test_templates")
+        template_dir=TEST_TEMPLATE_DIRS, config={"base_url": "/prefix/"}
     ) as client:
         yield client
 
@@ -23,6 +31,12 @@ def test_request_is_available(custom_pages_client):
     assert "path:/request" == response.text
 
 
+def test_custom_pages_with_base_url(custom_pages_client_with_base_url):
+    response = custom_pages_client_with_base_url.get("/prefix/request")
+    assert 200 == response.status
+    assert "path:/prefix/request" == response.text
+
+
 def test_custom_pages_nested(custom_pages_client):
     response = custom_pages_client.get("/nested/nest")
     assert 200 == response.status

tests/test_html.py

@@ -1523,6 +1523,7 @@ def test_base_url_config(app_client_base_url_prefix, path):
             and href
             not in {
                 "https://datasette.io/",
+                "https://github.com/simonw/datasette",
                 "https://github.com/simonw/datasette/blob/main/LICENSE",
                 "https://github.com/simonw/datasette/blob/main/tests/fixtures.py",
                 "/login-as-root",  # Only used for the latest.datasette.io demo