clarify support for node 12 and 14

* run node 12, 14, and 16 (next to be supported) on CI. This will catch
  node version-specific problems like that solved in dad9bdc.
* mention 12 and 14 in build instructions and link to instructions for
  installing that version.
* follow this in Dockerfile, installing version 14. This addresses
  a "Cannot find module 'worker_threads'" error introduced in
  39a63e0, which (inadvisedly) upgraded gzipper 4->5 in addition to
  the material-ui upgrade.
* use utf-8 encoding rather than ascii in live part parser. Those
  builds apparently don't support ascii. iThey must use "small-icu" or
  have ICU disabled, as described here:
  https://nodejs.org/api/util.html#util_encodings_supported_when_node_js_is_built_with_the_small_icu_option

.github/workflows/ci.yml

@@ -8,13 +8,10 @@ env:
 
 jobs:
   rust:
-    name: Test Rust server code
+    name: Rust ${{ matrix.rust }}
     strategy:
       matrix:
-        rust:
-          - stable
-          - 1.52
-          - nightly
+        rust: [ "stable", "1.52", "nightly" ]
         include:
           - rust: nightly
             extra_args: "--features nightly --benches"
@@ -47,17 +44,16 @@ jobs:
         if: matrix.rust == 'stable'
         run: cd server && cargo fmt --all -- --check
   js:
-    name: Build, test, lint, and check formatting of Javascript frontend
+    name: Node ${{ matrix.node }}
+    strategy:
+      matrix:
+        node: [ "12", "14", "16", "18" ]
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2
         with:
-          node-version: "14"
-          path: ~/.npm
-          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
-          restore-keys: |
-            ${{ runner.os }}-node-
+          node-version: ${{ matrix.node }}
       - run: cd ui && npm ci
       - run: cd ui && npm run build
       - run: cd ui && npm run test