startup: use "npm ci" instead of "npm install" in startup scripts

Advantages:
- reproducible install: every user will have the same, exact install, instead of
  a slightly different one
- speed: installation of dependencies is measurably faster
- explicit: if a user setup is broken, from now on he'll have a clear error
  message

Fixes #3778

bin/installDeps.sh

@@ -100,7 +100,7 @@ echo "Ensure that all dependencies are up to date...  If this is the first time
   cd node_modules
   [ -e ep_etherpad-lite ] || ln -s ../src ep_etherpad-lite
   cd ep_etherpad-lite
-  npm install --save --loglevel warn
+  npm ci
 ) || {
   rm -rf src/node_modules
   exit 1