[#58148980] First crack at Deb build+package

Shell script to be called by Jenkins which will:

- Create a temporary local directory for Go to fetch dependencies.
- Call `go get` for projects deps.
- Call `go build` to produce a binary.
- Install `fpm` using Bundler.
- Create a Deb containing the single binary under `/usr/bin`.

Some things that need further consideration:

- The triemux dep which lives within the same repo and gets cloned
  separately into `$GOPATH` by `go get`.
- When does Jenkins tag the repo with a build number.
- Push the `.deb` to an APT repo.
[#58148980] Symlink router into GOPATH

This prevents a second (unversioned) download of the router repo in order to
satisfy the triemux dependency.
[#58148980] Don't install deps with go get

This seems to be the "done thing". Should be slightly faster?
[#58148980] Use BUILD_NUMBER for FPM package

This will be a plain integer from Jenkins.
[#58148980] Install bundle to central location

Will prevent FPM's dependencies from being downloaded on every run. We want
this outside of the workspace, since we delete the workspace on every build.
Merge pull request #2 from alphagov/58148980-packaging

[#58148980] build+package