fix: urls.primaryName functionality regression (via #5097)

* add tests

* compute index before triggering URL load

* bonus: improve urls topbar label

test/e2e-cypress/static/configs/urls-primary-name.yaml

@@ -0,0 +1,6 @@
+urls:
+- name: One
+  url: /documents/features/urls/1.yaml
+- name: Two
+  url: /documents/features/urls/2.yaml
+urls.primaryName: Two

test/e2e-cypress/static/configs/urls.yaml

@@ -0,0 +1,5 @@
+urls:
+- name: One
+  url: /documents/features/urls/1.yaml
+- name: Two
+  url: /documents/features/urls/2.yaml

test/e2e-cypress/static/documents/features/urls/1.yaml

@@ -0,0 +1,12 @@
+swagger: "2.0"
+
+info:
+  title: One
+
+paths:
+  /:
+    get:
+      summary: an operation
+      responses:
+        "200":
+          description: OK

test/e2e-cypress/static/documents/features/urls/2.yaml

@@ -0,0 +1,12 @@
+openapi: "3.0.0"
+
+info:
+  title: Two
+
+paths:
+  /:
+    get:
+      summary: an operation
+      responses:
+        "200":
+          description: OK