Update CI GitHub workflow

Allow PRs to checkout non develop branch from core Alaveteli. This is
useful for PRs which depend on core changes in order to work.

.github/workflows/ci.yml

@@ -40,13 +40,35 @@ jobs:
     env:
       DATABASE_URL: postgres://postgres:postgres@localhost:5432/alaveteli_test
       RAILS_ENV: test
+      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
     steps:
+    - name: Get theme PR description
+      id: pr_theme_description
+      run: |
+        PR_DESCRIPTION=$(gh pr view https://github.com/mysociety/whatdotheyknow-theme/pull/${{ github.event.pull_request.number }} --json body -q '.body')
+        echo $PR_DESCRIPTION
+        echo "PR_DESCRIPTION=$PR_DESCRIPTION" >> $GITHUB_ENV
+
+    - name: Extract associated required core PR number
+      id: pr_core_number
+      run: |
+        echo "PR_CORE_NUMBER=$(echo $PR_DESCRIPTION | grep -o 'https://github.com/mysociety/alaveteli/pull/[0-9]*' | grep -o '[0-9]*$')" >> $GITHUB_ENV
+
+    - name: Get core branch name
+      id: pr_core_branch
+      run: |
+        if [[ ! -z "$PR_CORE_NUMBER" ]]; then
+          echo "PR_CORE_BRANCH=$(gh pr view https://github.com/mysociety/alaveteli/pull/$PR_CORE_NUMBER --json headRefName -q '.headRefName')" >> $GITHUB_ENV
+        else
+          echo "PR_CORE_BRANCH=develop" >> $GITHUB_ENV
+        fi
+
     - name: Checkout Alaveteli
       uses: actions/checkout@v2
       with:
         repository: mysociety/alaveteli
-        ref: develop
+        ref: $PR_CORE_BRANCH
         path: core
         submodules: true
         fetch-depth: 0