Automatically lock closed issues/prs after 90 days

We've been getting a bunch of comments on old closed issues. Instead
people should create new ones.

This adds a github "workflow" that should lock closed issues/prs after
90 days, except those labelled "question".

Let's see how it works out.

.github/workflows/lockthreads.yml

@@ -0,0 +1,16 @@
+name: 'Lock threads'
+
+on:
+  schedule:
+    - cron: '0 * * * *'
+
+jobs:
+  lock:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/lock-threads@v2
+        with:
+          github-token: ${{ github.token }}
+          issue-lock-inactive-days: '90'
+          pr-lock-inactive-days: '90'
+          issue-exclude-labels: 'question'