Run make lint job in GitHub Actions

This is a ground-up rewrite of the "lint" job instead of porting it from
CircleCI. It will be much faster since it doesn't try to spin up a
Docker container and work through our various unnecessary caching
layers. It does literally the bare minimum to run lints.

The CircleCI lint job needs to remain for now because it's super
intertwined with everything else.

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+on: [push, pull_request]
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    container: ubuntu:focal
+    steps:
+      - name: Install Git
+        run: |
+          apt-get update && apt-get install --yes git
+      - uses: actions/checkout@v3
+      - name: Install dependencies
+        run: |
+          DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt-get install --yes --no-install-recommends \
+            build-essential python3-virtualenv python3-dev enchant file apache2-dev
+          virtualenv .venv
+          # TODO: this should be one step, but there are too many conflicting dependencies
+          ./.venv/bin/pip install -r securedrop/requirements/python3/test-requirements.txt
+          ./.venv/bin/pip install -r securedrop/requirements/python3/requirements.txt
+          ./.venv/bin/pip install -r securedrop/requirements/python3/develop-requirements.txt
+      - name: Run lint
+        run: |
+          git config --global --add safe.directory $GITHUB_WORKSPACE
+          source .venv/bin/activate
+          make lint