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 +40,40 @@ 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=$PR_DESCRIPTION" >> $GITHUB_ENV
+      shell: bash
+
+    - name: Extract associated required core PR
+      id: pr_core_url
+      run: |
+        PR_CORE_URL=$(echo "$PR_DESCRIPTION" | grep -o 'https://github.com/mysociety/alaveteli/pull/[0-9]*')
+        echo "PR_CORE_URL=$PR_CORE_URL" >> $GITHUB_ENV
+      shell: bash
+
+    - name: Get core branch name
+      id: pr_core_branch
+      run: |
+        if [[ ! -z "$PR_CORE_URL" ]]; then
+          PR_NUMBER=$(echo "$PR_CORE_URL" | grep -o '[0-9]*$')
+          CORE_BRANCH_NAME=$(gh pr view https://github.com/mysociety/alaveteli/pull/$PR_NUMBER --json headRefName -q '.headRefName')
+          echo "CORE_BRANCH_NAME=$CORE_BRANCH_NAME" >> $GITHUB_ENV
+        else
+          echo "CORE_BRANCH_NAME=develop" >> $GITHUB_ENV
+        fi
+      shell: bash
+
     - name: Checkout Alaveteli
       uses: actions/checkout@v2
       with:
         repository: mysociety/alaveteli
-        ref: develop
+        ref: $CORE_BRANCH_NAME
         path: core
         submodules: true
         fetch-depth: 0