GitHub integration: Pull upstream to fork before creating a PR

[adamziel]

Let's synchronize the upstream repo to personal fork before creating a PR via the "Export to GitHub" modal. Otherwise we'll soon start seeing conflicts in fresh PRs. There's some code to "synchronize the fork" at octokit/octokit.net#1833. I'm worried that could be slow, but slow is better than broken.

Context:

• Unexpected error when exporting to Github #1367
• GitHub export: Create new commits in your fork when writing to the upstream repo isn't allowed #1392 (comment)