* Return locks when they're deleted
* Implement DeletePlan and DeletePlanByPull
* Clean up data from pull request on close
* Delete plan on successful apply
* set version in one place
* rename ExecutionContext to CommandContext
* delete unneeded PullRequestContext since it just duplicated what is in CommandContext
* split fields inside CommandContext into Repo, PullRequest, and User models
* clean up unused fields
* remove base_executor stuff which was trying to do inheritance in go and didn't get us anywhere
* clean up unused code and assets
* created a locking.Client that creates the Keys that are used on the front-end. This allows the backends to store the locks any way they like as long as they support the interface
