Support for ad-hoc deployments

[timperrett]

Frequently users would like to be able to create deployments from pull-requests, or arbitrary branches that are not mainline code. In order to support these non-release deployment use cases, we propose that these ad-hoc deployments create subordinate namespaces (ad-hoc namespaces) and publish their deployment there.

For example, consider a pull request, upon commenting on the PR with something like nelson deploy, Nelson could get a callback webhook and then derive metadata about that PR (e.g. its name, id etc) and create a namespace dev/adhoc/yourproject-193, then launching the stack into this namespace.