Add a CI test for unbuilt assets

Many of our PRs have out of date assets, making it hard to test the
latest changes. This will at least throw an error on the PR when build
assets aren't updated.

.circleci/config.yml

@@ -1,5 +1,8 @@
 version: 2.1
 
+orbs:
+  untracked_changes: niteo/check-untracked-changes@1.0.0
+
 commands:
   run-tox:
     description: "Run tox"
@@ -13,8 +16,21 @@ commands:
       - checkout
       - run: pip install --user tox
       - run: tox -e "<<parameters.version>>-sphinx{<<parameters.sphinx-version>>}"
+  run-build:
+    description: "Verify assets were built"
+    steps:
+      - checkout
+      - run: npm ci
+      - run: npm run build
+      - untracked_changes/check:
+          path: sphinx_rtd_theme/
 
 jobs:
+  build:
+    docker:
+      - image: 'cimg/python:3.9-node'
+    steps:
+      - run-build: {}
   py27:
     docker:
       - image: 'cimg/python:2.7'
@@ -50,6 +66,7 @@ workflows:
   version: 2
   tests:
     jobs:
+      - build
       - py39
       - py38
       - py37