Add frontend to search rebuild script

Frontend indexes the /help page

There is currently also a panopticon:register rake task in frontend,
but @alext indicates that this almost certainly shouldn't exist.
Support multiple tasks per app

Travel-advice-publisher will need this.
Add travel-advice to search rebuild script
Add designprinciples/service-manual to search rebuild
Merge pull request #3 from alphagov/add-missing-apps

Add missing apps to search rebuild script