Add static analysis job to find invalid YAML in changelogs

When a changelog has invalid YAML (typically, there is an unquoted @ at the
start of the author field), then the entry will be discarded. This script checks
all unreleased changelogs for validity, and runs as part of the static-analysis
step, so the pipeline will fail if this happens in future.

changelogs/unreleased/lint-changelog-yaml.yml

+---
+title: Detect when changelog entries are invalid
+merge_request:
+author:
+type: other

scripts/lint-changelog-yaml

+#!/usr/bin/env ruby
+
+require 'yaml'
+
+invalid_changelogs = Dir['changelogs/**/*.yml'].reject do |changelog|
+  begin
+    YAML.load_file(changelog)
+  rescue
+  end
+end
+
+if invalid_changelogs.any?
+  puts "Changelogs with invalid YAML found!\n"
+  puts invalid_changelogs.sort
+  exit 1
+else
+  puts "All changelogs are valid YAML.\n"
+  exit 0
+end

scripts/static-analysis

@@ -13,7 +13,8 @@ tasks = [
   %w[yarn run eslint],
   %w[bundle exec rubocop --require rubocop-rspec],
   %w[scripts/lint-conflicts.sh],
-  %w[bundle exec rake gettext:lint]
+  %w[bundle exec rake gettext:lint],
+  %w[scripts/lint-changelog-yaml]
 ]
 
 failed_tasks = tasks.reduce({}) do |failures, task|