Clearer repository layout; clearer distinction between dev and prod playbooks; better documentation

[benarmston]

This PR reorganises the repository structure to make things more obvious. In particular, it aims to be

• clearer to admins deploying concertim what ansible playbook they should be running.
• clearer to devs developing the production ansible playbook, how to test it.
• clearer to devs developing mrd and vis-app how they should set up their development environment.

It does that with the following changes:

• Rename vagrant directory to contrib/dev/vagrant - the vagrant machine is only used for development, so let's hide it out of the way.
• Rename ansible directory to contrib/dev/ansible - this is now a dev-only ansible playbook.
• Change the contrib/dev/ansible playbook to be suitable only for development.
  • No longer build, store and fetch assets in/from S3.
  • No longer install "production" versions of mrd and vis-app.
  • Always install "development" versions of mrd and vis-app.
  • Remove monit - there is no need to monitor dev servers.
• Rename production directory to ansible - this is the production-only ansible playbook.

The documentation has also seen improvements.