Add a bunch of CI tests for the code

.github/workflows/main.yaml

@@ -0,0 +1,50 @@
+name: Tests
+
+on:
+  push:
+    branches:
+      - master
+      - "release/*"
+  pull_request:
+    branches:
+      - master
+      - "release/*"
+  workflow_dispatch:
+
+jobs:
+  mypy:
+    runs-on: ubuntu-latest
+
+    name: 'typecheck: mypy'
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.9'
+      - name: Install
+        run: |
+          set -euo pipefail
+          pip install mypy
+          pip install -r requirements.txt
+      - name: Typecheck
+        run: |
+          python -m mypy --config mypy.ini -p snikket_web
+
+  linting:
+    runs-on: ubuntu-latest
+
+    name: 'lint: flake8'
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.9'
+      - name: Install
+        run: |
+          set -euo pipefail
+          pip install flake8
+      - name: Linting
+        run: |
+          python -m flake8 snikket_web