Add experimental Gitlab CI integration

Based on the configuration recommended by Knapsack PRO [1] and Pronto
[2].

So far there are a few issues:

* Bundled gems don't seem to be cached
* The Ruby version can't be read from .ruby-version and needs to be set
  manually
* Tests are slower than when run on Github Actions or Travis CI
* There seem to be many flaky tests, maybe because tests are slower
  (this might actually help us fixing flaky bugs or performance
  bottlenecks)

An advantage of Gitlab CI is we can access the screenshots taken when a
system test fails.

[1] https://docs.knapsackpro.com/2019/how-to-run-parallel-jobs-for-rspec-tests-on-gitlab-ci-pipeline-and-speed-up-ruby-javascript-testing
[2] https://github.com/prontolabs/pronto/#gitlab-integration

.gitlab-ci.yml

@@ -0,0 +1,49 @@
+stages:
+  - test
+  - lint
+
+tests:
+  image: "ruby:2.6.6"
+  stage: test
+  services:
+    - postgres:10.10
+  cache:
+    key: consul
+    paths:
+      - vendor/
+  variables:
+    POSTGRES_USER: consul
+    RAILS_ENV: test
+    COVERALLS: 1
+  parallel: 5
+  script:
+    - apt-get update && apt-get install -y nodejs chromium
+    - for i in config/*.example; do cp "$i" "${i/.example}"; done
+    - for i in config/*.gitlab; do cp "$i" "${i/.gitlab}"; done
+    - bundle --without development
+    - bundle exec rake db:setup
+    - bundle exec rake assets:precompile > /dev/null 2>&1
+    - bin/knapsack_pro_rspec
+  artifacts:
+    when: on_failure
+    paths:
+      - tmp/screenshots/
+
+# To make this job work, create a Personal Access Token with permissions
+# to comment on your repository and add a variable named
+# PRONTO_GITLAB_API_PRIVATE_TOKEN to your repository CI/CD settings
+# giving it the value of the Personal Access Token
+linters:
+  image: "ruby:2.6.6"
+  stage: lint
+  cache:
+    key: consul
+    paths:
+      - vendor/
+  only:
+    - merge_requests
+  script:
+    - apt-get update && apt-get install -y nodejs cmake pkg-config
+    - bundle --without test
+    - git fetch origin $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
+    - bundle exec pronto run -f gitlab_mr -c origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME

config/database.yml.gitlab

@@ -0,0 +1,11 @@
+default: &default
+  adapter: postgresql
+  encoding: unicode
+  host: postgres
+  pool: 5
+  username: consul
+  password:
+
+test:
+  <<: *default
+  database: consul_test