Pull Requests

Summary: This PR adds basic pull request functionality through Elodin -- you can specify a target branch and are prompted to describe the PR using a template. Validations and side checks (pushing unpushed commits, etc.) are not yet implemented but will come in a follow-up diff.

Test Plan: dogfood!

Reviewer:
  @baroo please
  @gnarmis please
Need to rename this to Shep