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

docker/dev-common.bash

@@ -32,7 +32,6 @@ packages+=(
     curl
     pkgconf
     locales
-    npm
     sudo
     sqlite3
     tzdata
@@ -41,6 +40,10 @@ packages+=(
 time apt-get update
 time apt-get install --assume-yes --no-install-recommends "${packages[@]}"
 
+# Install a more recent nodejs/npm than in the universe repository.
+time curl -sL https://deb.nodesource.com/setup_14.x | bash -
+time apt-get install nodejs
+
 # Create the user. On the dev environment, allow sudo.
 groupadd \
     --gid="${BUILD_GID}" \
@@ -55,7 +58,6 @@ useradd \
     moonfire-nvr
 echo 'moonfire-nvr ALL=(ALL) NOPASSWD: ALL' >>/etc/sudoers
 
-
 # Install Rust. Note curl was already installed for yarn above.
 time su moonfire-nvr -lc "
     curl --proto =https --tlsv1.2 -sSf https://sh.rustup.rs |