Resolve Issue 3114 "Post-install scan of duplicate Sprocket manifest files"

Under issue #3114 (closed), it was discovered that when reinstalling GitLab after a failed install, previous Sprocket manifest files may have been left on the filesystem. Having multiple sprocket files can cause unexpected results when using GitLab.

This merge is for changes to the post-install script (postinst.erb) file, adding a function that checks the number of files in the ${DEST_DIR}/embedded/service/gitlab-rails/public/assets/ directory. If the number of files is higher than one, a warning message will be displayed, advising the user of the multiple files, and request that they clean up the older Sprocket files. The full name and path of the extraneous files will be given to the user (minus the latest installed copy of the file), so the user will know what needs to be removed.