Server: add automatic security auditing for PR/main.

We can't merge it until #284 is fixed.

.github/workflows/server-security.yml

@@ -0,0 +1,22 @@
+name: Server Security
+
+on:
+  push:
+    branches:
+    - main
+    paths:
+      - '**/Cargo.toml'
+      - '**/Cargo.lock'
+  pull_request:
+    paths:
+      - '**/Cargo.toml'
+      - '**/Cargo.lock'
+
+jobs:
+  security_audit:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - uses: actions-rs/audit-check@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}