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