a8n: Backend stuck on npm campaign creation

Created by: eseliger

• Sourcegraph version: 3.11, master
• Platform information: Local Enterprise Dev, macOS Catalina

Steps to reproduce:

1. Create a new campaign, input

   {
       "scopeQuery": "repo:github.com/sourcegraph/automation-testing",
       "matchers": [{ "type": "npm" }]
   }

2. Create preview
3. Hit "Create"
4. Wait forever

Expected behavior:

It finishes at some point creating the PR on GH.com

Actual behavior:

It never finishes, and the backend seems to be busy, not creating any other previews and so forth either, this could only be solved by restarting the server