Add a simulate page

[ashmeer7]

This allows a user to:

• Input a branch that has a policy file. The simulate page will evaluate that policy file on the given PR
• In put a user to simulate a fake approval for. The simulate page will evaluate the PR as if the user had submitted an approval
• Or both!

Abstracted away the majority of functionality into a BaseDetails handler. This allows consumers to submit a policy and a user to override the defaults done by the Details handler. The Details page still exists since it currently sends the information back to GitHub.

Here is a sample of the simulate page on a test repository:

[bluekeyes]

Could you add a screenshot of what the simulate form and the button look like to the PR description?

[bluekeyes]

#476 is going to conflict with this, but it should make it much easier to evaluate a PR without posting status checks.

[ashmeer7]

Addressed some PR comments and fixed what I could fix without getting a whole test setup. I'll be back with more questions soon enough.

[elliot-graebert-skydio]

@bluekeyes this came up when deploying Policy-Bot. Could we get this merged? Use case was simply trying to test out a new policy on a branch, and not knowing the right path to do so.