Tracking deltas from upstream coreos/dex

[vito]

It looks like we'll be sitting on this fork for a while, so I'm gonna try and keep track of all our PR-able changes here before things get too hairy.

I'm also gonna replace all the import paths from coreos/dex to concourse/dex and make this a proper fork until we can get those PRs merged. We're relying on submodules in the concourse/concourse repo for this import path repointing, but that won't work with Go 1.11's modules, which I'd like to start using.

Here's the plan:

1. Create a new branch called upstream which will point to the coreos/dex upstream master that all local branches are rebased on.
2. Create new branches for reach of our PR-able changes, based on upstream. This way they can be submitted individually rather than all being merged at once.
3. Reset master to upstream master.
4. Delete all branches but master and the local PR branches (we inherited a lot of'em from upstream and it's confusing to keep them).
5. Maintain each local branch for as long as needed to get merged upstream (this may involve rebasing on upstream and force-pushing).
6. Maintain master as one big merge of all local branches and on top of upstream (this will definitely involve force-pushing, as local branches are constantly rebased).
7. Any time we re-push master, put a commit on top that re-points all coreos/dex import paths to concourse/dex.
8. At the same time, tag HEAD with something so that the ref doesn't go away (thereby breaking old submodule pointers in concourse/concourse).

The downside of this is that master will be force-pushed all the time. The upside is that go get should "just work", all import paths will point at our fork, and all our changes will be PR-able.