Create semgrep.yml (#294)

## What was changed
Add Github workflow to scan PRs with Semgrep.

## Why?
This will eventually be an org-wide required workflow, but Github won't let us have access to it yet, so this PR manually adds scanning.

## Checklist
How was this tested:

Added manually in this way to many other repos with no issue.

.github/workflows/semgrep.yml

@@ -0,0 +1,25 @@
+name: Semgrep
+on:
+  workflow_dispatch: {}
+  pull_request: {}
+  push:
+    branches:
+      - main
+      - master
+    paths:
+      - .github/workflows/semgrep.yml
+  schedule:
+    # random HH:MM to avoid a load spike on GitHub Actions at 00:00
+    - cron: '15 21 * * *'
+jobs:
+  semgrep:
+    name: semgrep/ci
+    runs-on: ubuntu-20.04
+    env:
+      SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
+    container:
+      image: returntocorp/semgrep
+    if: (github.actor != 'dependabot[bot]')
+    steps:
+      - uses: actions/checkout@v3
+      - run: semgrep ci