Put the Jenkins tasks into a shell script

This can then be run by any of the jobs, and creates artefacts that can
be deployed to all the environments, if they build successfully.

To run successfully, Jenkins needs:
- SSH access to the Github repo
- Submodule processing tickbox not ticked
- SSH agent tickbox ticked, with the appropriate user selected
- Cookie encryption key should be specified as a bash variable before running
  this script
- Git publisher set up to push $BUILD_TAG as a new Git tag to Github (optional,
  but should be used for so we look on Github and see exactly what's in each
  build)

For an example of a Jenkins job, see Supplier-Submission-Build
Merge pull request #85 from alphagov/put-jenkins-into-script

Put the Jenkins tasks into a shell script