Move CI to Jenkins

Currently we use Harbormaster to build Differentials and commits. While this works, it leaves much to be desired:

• job scheduling is (literally) random: this means that some patch authors end up waiting literally ways for their patches to be built
• there is little support for automatic provisioning of builders: this means we can't scale to meet demand and make poor use of our computational resources
• periodic builds (e.g. nightlies) are not supported.
• it lacks a sensible interface for integration with external tools: this means that efforts like CI-before-merge have been pushed off
• status reporting is poor: even answering the question of what a given builder is currently working on is surprisingly difficult
• common maintenance tasks are byzantine: Adding a new builder requires adding at least six different objects (a Harbormaster Build Plan, a Drydock Working Copy, a Drydock Blueprint, an Almanac Service, an Almanac Device, and an Almanac Binding) in various Phabricator applications. None of this configuration can be tracked under version control nor can most of it be cloned from an existing builder's configuration.
• Design assumptions don't match GHC's constraints: Harbormaster was designed under the assumption that builds are cheap and computation plentiful. The maintainers have stated that they have little interest in supporting environments where this doesn't hold.

Jenkins, while far from perfect, seems a bit better suited to our needs, more mature, and far more flexible. This ticket will serve as the checklist for our move to Jenkins.

See Ben's blog post (Aug 17).

In the end we want,

• Builders (static or dynamically-provisioned, as appropriate) for

  * x86-64, i386 Linux

  * x86-64, i386 Windows

  * x86-64 Darwin

  * x86-64 OpenBSD

  * Cross compile from x86-64 to ARM

  * Native ARM

• Differential builds with sensible scheduling (e.g. first build on 64-bit Linux where machines are cheap, then build on the others)

• Per-commit builds on all

• Nightly builds on all, including

  * Collection of binary distribution for user download

  * Update of master documentation mirror on downloads.haskell.org

  * Slow validation (including tests requiring Hackage packages)

  * nofib run on some platforms?

• Test-before-merge-to-master