Add kanban automation for new pull requests

So now, newly opened pull requests will automatically be added to the
"Reviewing" column, while newly opened draft pull requests will
automatically be added to the "Doing" column.

I've added the "reopened" event just in case (and was indeed very useful
while testing this feature), although we rarely reopen pull requests.

Note that this only works on pull requests that aren't already in the
project; that is, if a pull request is already in the "Doing" column,
closing it and reopening it will *not* move it to the "Reviewing
column".

So it looks like we won't easily be able to extend this feature in order
to automatically move pull requests when they're marked as ready for
review.

.github/workflows/kanban.yml

@@ -0,0 +1,25 @@
+name: Assign new pull requests
+on:
+  pull_request_target:
+    types: [opened, reopened]
+env:
+  MY_GITHUB_TOKEN: ${{ secrets.KANBAN_TOKEN }}
+
+jobs:
+  assign_one_project:
+    runs-on: ubuntu-latest
+    name: Assign new pull requests
+    steps:
+    - name: Assign new pull requests to the reviewing column
+      uses: srggrs/assign-one-project-github-action@1.3.1
+      if: github.event.pull_request.draft == false
+      with:
+        project: "https://github.com/orgs/consul/projects/1"
+        column_name: "Reviewing"
+
+    - name: Assign new draft pull requests to the doing column
+      uses: srggrs/assign-one-project-github-action@1.3.1
+      if: github.event.pull_request.draft == true
+      with:
+        project: "https://github.com/orgs/consul/projects/1"
+        column_name: "Doing"