switch from yarn to npm

This eases build setup. Where Yarn requires a separate package
repository, npm is available in the standard one.

yarn's package repository signature was recently expired, and apparently
will expire again in a year. Avoid dealing with that.

Fixes #110.

.github/workflows/ci.yml

@@ -47,16 +47,10 @@ jobs:
     - uses: actions/setup-node@v2
       with:
         node-version: '14'
-    - name: Get yarn cache directory path
-      id: yarn-cache-dir-path
-      run: echo "::set-output name=dir::$(yarn cache dir)"
-    - uses: actions/cache@v2
-      id: yarn-cache # use this to check for `cache-hit` (`steps.yarn-cache.outputs.cache-hit != 'true'`)
-      with:
-        path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
-        key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+        path: ~/.npm
+        key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
         restore-keys: |
-          ${{ runner.os }}-yarn-
-    - run: cd ui && yarn install
-    - run: cd ui && yarn build
-    - run: cd ui && yarn lint
+          ${{ runner.os }}-node-
+    - run: cd ui && npm ci
+    - run: cd ui && npm run build
+    - run: cd ui && npm run lint