CI: Add GitHub workflow to check for readable.css updates (#15)

This adds a workflow and accompanying script to update readable.css. The
script gets the latest tag version from the readable.css repository on
codeberg.org and compares it to the currently used version. If the
versions don't match up, we download readable.css and readable.min.css
from Codeberg and update layouts/partials/head.html.

.github/scripts/update-readable-css.sh

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+#!/bin/bash
+set -o errexit
+set -o pipefail
+set -o nounset
+
+# Query the codeberg.org API (see
+# https://codeberg.org/api/swagger#/repository/repoListReleases) and use jq to
+# get the tag name of the latest release (i.e. the first element of the array
+# in the JSON output). Finally, use sed to remove the double quotes around the
+# value.
+TAG_VERSION=$(curl -s \
+    -H 'accept: application/json' \
+    https://codeberg.org/api/v1/repos/Freedom-to-Write/readable.css/releases \
+    | jq '.[0].tag_name' \
+    | sed 's/"//g')
+
+echo "Latest tag: ${TAG_VERSION}"
+
+# Get the current version.
+# The '-o' option will only output the match grep found.
+# We look for readable.min.css and a set of three numbers seperated by dots,
+# i.e. a semantic version. readable.min.css is used to make sure that we match
+# the right numbers in the file.
+# We use sed to get rid of the readable.min.css prefix.
+CURRENT_VERSION=$(grep -o \
+    'readable.min.css?v=[[:digit:]]\+\.[[:digit:]]\+\.[[:digit:]]\+' \
+    layouts/partials/head.html \
+    | sed 's/readable.min.css?v=/v/')
+echo "Current version: ${CURRENT_VERSION}"
+
+if [[ $TAG_VERSION == $CURRENT_VERSION ]]
+then
+    echo "Nothing to do. The current version is already up to date."
+else
+    curl -s "https://codeberg.org/Freedom-to-Write/readable.css/raw/tag/${TAG_VERSION}/readable.css" > static/css/readable.css
+    curl -s "https://codeberg.org/Freedom-to-Write/readable.css/raw/tag/${TAG_VERSION}/readable.min.css" > static/css/readable.min.css
+    VERSION=$(echo "${TAG_VERSION}" | sed 's/v//')
+    sed -i "s/readable.min.css?v=.*/readable.min.css?v=${VERSION}\">/" layouts/partials/head.html
+fi
+
+# Add the latest tag version to the workflow environment, so we can access
+# it in later steps.
+echo "READABLE_CSS_TAG=${TAG_VERSION}" >> "$GITHUB_ENV"

.github/workflows/update-readable-css.yml

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+name: update-readable-css
+
+on:
+  schedule:
+    # Every day at 3:28.
+    - cron: '28 3 * * *'
+
+jobs:
+  update-readable-css:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Download CSS files for latest tag
+        run: .github/scripts/update-readable-css.sh
+      - name: Create pull request if files have changed
+        # https://github.com/marketplace/actions/create-pull-request
+        uses: peter-evans/create-pull-request@v5
+        with:
+          # First line is the commit subject as always. The rest goes
+          # into the body.
+          commit-message: |
+            Update readable.css to ${{ env.READABLE_CSS_TAG }}
+
+            See the changelog here:
+
+            https://codeberg.org/Freedom-to-Write/readable.css/src/tag/${{ env.READABLE_CSS_TAG }}/CHANGELOG.md
+          branch: update-readable-css
+          delete-branch: true
+          # Use 'GitHub' both times.
+          # This is already the default for committer, but the author defaults
+          # to the user who triggered the workflow run, which is the owner of
+          # the repository.
+          # We use the same value for the author to indicate that the
+          # commit was created by a bot.
+          committer: GitHub <noreply@github.com>
+          author: GitHub <noreply@github.com>
+          title: Update readable.css to ${{ env.READABLE_CSS_TAG }}
+          body: |
+            See the changelog here:
+
+            https://codeberg.org/Freedom-to-Write/readable.css/src/tag/${{ env.READABLE_CSS_TAG }}/CHANGELOG.md
+          labels: update-readable-css