Clean up Jenkins build output

[fchavant]

Expected behaviour

• The output of npm run dev (stdout and stderr) is redirected to a file;
• npm run is called with the --silent switch

Actual behaviour

• The output of npm run dev is not being redirected to a file;
• npm show is very verbose on errors

Steps to reproduce

Look the output of any failed build in Jenkins:

• the output of npm run dev can be seen
• npm shows unnecessarily verbose messages on errors