Merge branch '21814-fix' into 'master'

Pull import sources from Gitlab::ImportSources

## What does this MR do?
`ApplicationSettings.create_from_defaults` defines an array of import sources, but we should be pulling them from `Gitlab::ImportSources.options` since that's what they're validated against.

## What are the relevant issue numbers?

Closes #21814

See merge request !6293

app/models/application_setting.rb

@@ -146,7 +146,7 @@ class ApplicationSetting < ActiveRecord::Base
       default_project_visibility: Settings.gitlab.default_projects_features['visibility_level'],
       default_snippet_visibility: Settings.gitlab.default_projects_features['visibility_level'],
       domain_whitelist: Settings.gitlab['domain_whitelist'],
-      import_sources: %w[github bitbucket gitlab google_code fogbugz git gitlab_project],
+      import_sources: Gitlab::ImportSources.values,
       shared_runners_enabled: Settings.gitlab_ci['shared_runners_enabled'],
       max_artifacts_size: Settings.artifacts['max_size'],
       require_two_factor_authentication: false,