Allow try builds after approval

[bryanburgers]

Currently, once a pull request has been approved r+, it can not be tried.

There are times when we want to do a timer run, even after r+ing the merge request, so it would be great if we could still @bors try after approval.

cc @Centril

[bryanburgers]

The current architecture of homu is that we only track a single run state, which is modified by a single boolean try_ on the pull request. That means that the run state is either the run state of a build run OR the run state of a try run.

This is the same decision that makes things like #47 happen. In between a try run starting and ending, the try_ flag gets flipped from true to false, so the homu interprets the run result as a build run result instead of try run result.

It affects us here because trying after approval will mean we need to set the try_ bit back on, and homu will get all confused if it then starts a build run. That's why #3 was fixed.

In the future, we can move toward storing a separate production state and try state, so that these runs can happen in parallel without confusing the pull request state. I plan on including that change in #45 (whenever I can find the time to get back around to it).