Show message and don't start if javascript files have not been built

fixes #1782
- added builtFilesExist function to check for files during startup.
  If built files do not exist Ghost startup is stopped and a link
  to the documentation is displayed.
- exported a scriptFiles object from server/helpers.
- added a builtScriptPath to the paths module.
- removed "js-msg" about missing javascript from the UI.
Merge pull request #1956 from jaswilli/issue-1782

Show message and don't start if javascript files have not been built