Verify the schema.rb integrity when running our CI

In the past, we've run into situations where we commited a version of
the schema.rb to version control which wasn't in sync with what you get
when you create the database and run the migrations. This is something
that might happen if you're working on different branches at the same
time, run the migrations in all those branches, and then commit the
current status of your database to your current branch. That will result
in a schema file which contains the database changes done in all these
branches instead of just the current one.

Although this hasn't happened to us for years, we see it happening
sometimes in other COSUL installations, and there's always a chance that
we make a mistake and do it again.

Until now, it wasn't a big deal because the installer sets up the
database by running `db:create db:migrate`, which runs the migrations
instead of using the `schema.rb` file.

However, we're going to add multitenancy using the Aparment gem, and
this gem uses the `schema.rb` file when creating a new schema for a new
tenant [1]. If the `schema.rb` file contains changes that shouldn't be
there, this will lead to different database schemas having different
tables and/or columns and future migrations might crash on production
when applied to some of these schemas. In other words, this mistake
could now result in a disastrous situation.

To help preventing that, we're adding a CI workflow that checks the
current `schema.rb` file is in sync with the database migrations.

[1] https://medium.com/infinite-monkeys/our-multi-tenancy-journey-with-postgres-schemas-and-apartment-6ecda151a21f#7e97

.github/workflows/db_schema.yml

@@ -0,0 +1,39 @@
+name: schema
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches-ignore:
+      - i18n_master
+
+permissions:
+  contents: read
+
+jobs:
+  schema:
+    runs-on: ubuntu-latest
+    services:
+      postgres:
+        image: postgres:10.10
+        ports: ["5432:5432"]
+        options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
+        env:
+          POSTGRES_USER: consul
+          POSTGRES_PASSWORD: ""
+    env:
+      PGUSER: consul
+      POSTGRES_HOST: postgres
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ruby/setup-ruby@v1
+        with:
+          bundler-cache: true
+      - name: Copy secrets and database files
+        run: for i in config/*.example; do cp "$i" "${i/.example}"; done
+      - name: Backup schema file
+        run: cp db/schema.rb db/commited_schema.rb
+      - name: Setup database
+        run: bundle exec rake db:create db:migrate
+      - name: Check the commited schema is correct
+        run: diff db/schema.rb db/commited_schema.rb