Change PR template format

[rtsao]

At first I thought only the PR titles were used, but now I just realized the body of a private PR will be in the synced commit message in the public repo.

The current template looks like:

<!-- DO NOT MODIFY THE FORMAT OF THIS BODY -->

## Summary

<!-- Replace this with your own summary -->
*No summary provided*

## Commit message overrides

<!--
  More info:
  https://github.com/uber-workflow/probot-app-usync#commit-messages

  Example:
  **fusionjs/fusionjs**
  ```
  My commit title

  My commit summary
  ```
-->

**owner/repo**

I propose changing the template to be something like:

<!-- DO NOT MODIFY THE FORMAT OF THIS BODY -->

## Public Summary

<!-- Replace this with your own summary -->
*No summary provided*

## Private Context

<!-- Replace this with private-only context -->
*No context provided*

With the existing format, I think it's more cumbersome to provide internal links/context. You are sort of forced to duplicate the public stuff from the summary into the public override. And PR titles should probably just never have anything private (always used as the commit title)