Rework Spack's continuous integration workflow to be environment-based.

- Add the `spack ci` command, which replaces the many scripts in `bin/`

- `spack ci` decouples the CI workflow from the spack instance:
  - CI is defined in a spack environment
  - environment is in its own (single) git repository, separate from Spack
  - spack instance used to run the pipeline is up to the user
  - A new `gitlab-ci` section in environments allows users to configure how
    specs in the environment should be mapped to runners
  - Compilers can be bootstrapped in the new pipeline workflow

- Add extensive documentation on pipelines (see `pipelines.rst` for further details)
- Add extensive tests for pipeline code