Tweak the Danger settings for DB changes

Instead of only checking for migrations, we now check for a variety of
files and directories that require a database review. We also include
some steps on how to make sure changes are reviewed.

danger/database/Dangerfile

+# frozen_string_literal: true
 # rubocop:disable Style/SignalException
 
-ANY_MIGRATIONS_REGEX = %r{\A(ee/)?(db/(geo/)?(post_)?migrate)/}
+# All the files/directories that should be reviewed by the DB team.
+DB_FILES = [
+  'db/',
+  'app/models/project_authorization.rb',
+  'app/services/users/refresh_authorized_projects_service.rb',
+  'lib/gitlab/background_migration.rb',
+  'lib/gitlab/background_migration/',
+  'lib/gitlab/database.rb',
+  'lib/gitlab/database/',
+  'lib/gitlab/github_import.rb',
+  'lib/gitlab/github_import/',
+  'lib/gitlab/sql/',
+  'rubocop/cop/migration',
+  'ee/db/',
+  'ee/lib/gitlab/database/'
+].freeze
+
 SCHEMA_NOT_UPDATED_MESSAGE = <<~MSG
 **New %<migrations>s added but %<schema>s wasn't updated.**
 
@@ -9,19 +26,28 @@ updated too (unless the migration isn't changing the DB schema
 and isn't the most recent one).
 MSG
 
+def database_paths_requiring_review(files)
+  to_review = []
+
+  files.each do |file|
+    review = DB_FILES.any? do |pattern|
+      file.start_with?(pattern)
+    end
+
+    to_review << file if review
+  end
+
+  to_review
+end
+
+all_files = git.added_files + git.modified_files
+
 non_geo_db_schema_updated = !git.modified_files.grep(%r{\Adb/schema\.rb/}).empty?
 geo_db_schema_updated = !git.modified_files.grep(%r{\Aee/db/geo/schema\.rb/}).empty?
 
-any_migration_created = !git.added_files.grep(ANY_MIGRATIONS_REGEX).empty?
-any_migration_updated = !git.modified_files.grep(ANY_MIGRATIONS_REGEX).empty?
-
 non_geo_migration_created = !git.added_files.grep(%r{\A(db/(post_)?migrate)/}).empty?
 geo_migration_created = !git.added_files.grep(%r{\Aee/db/geo/(post_)?migrate/}).empty?
 
-if any_migration_created || any_migration_updated || non_geo_db_schema_updated || geo_db_schema_updated
-  message "Please make sure to ask a Database engineer for a review."
-end
-
 if non_geo_migration_created && !non_geo_db_schema_updated
   warn format(SCHEMA_NOT_UPDATED_MESSAGE, migrations: 'migrations', schema: gitlab.html_link("db/schema.rb"))
 end
@@ -29,3 +55,30 @@ end
 if geo_migration_created && !geo_db_schema_updated
   warn format(SCHEMA_NOT_UPDATED_MESSAGE, migrations: 'Geo migrations', schema: gitlab.html_link("ee/db/geo/schema.rb"))
 end
+
+db_paths_to_review = database_paths_requiring_review(all_files)
+
+unless db_paths_to_review.empty?
+  message 'This merge request adds or changes files that require a ' \
+    'review from the Database team.'
+
+  markdown(<<~MARKDOWN.strip)
+## Database Review
+
+The following files require a review from the Database Team:
+
+* #{db_paths_to_review.join("\n* ")}
+
+To make sure these changes are reviewed, take the following steps:
+
+1. Edit your merge request, and add `gl-database` to the list of Group
+   approvers.
+1. Mention `@gl-database` in a separate comment, and explain what needs to be
+   reviewed by the team. Please don't mention the team until your changes are
+   ready for review.
+  MARKDOWN
+
+  unless gitlab.mr_labels.include?('database')
+    warn 'This merge request is missing the ~database label.'
+  end
+end