Add cleanup migration for MR's multiple assignees

See merge request gitlab-org/gitlab-ce!30269

Set the name of a user-namespace to the user name

See merge request gitlab-org/gitlab-ce!23272

Updates changed method names and fixes spec failures

Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

In order to avoid confusion between Gitlab::Importers module and other
like named modules, this renamed it to Gitlab::DatabaseImporters.

Remove CommonMetricsImporter patch and change all references to
CommonMetrics::Importer. Move specs into their appropriate folders. Also
cleans up some common_metric importer namespacing.

This moves our metric importers into the lib folder. This will allow
them to be autoloaded instead of having to explicitly require them.

This refactors common metrics importer for two reasons. 1. To create a
new pattern that can be followed by future importers that will minimize
dependency collision and 2. To allow EE to more easily extend
dependencies.

This fixes the `Namespace#name` and `Route#name` for all user
namespaces and their personal projects in case they don't match the
user name anymore.

More info info in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23272

The migration steals the remaining background jobs
of populating MRs with assignees, executes them
synchronously and then makes sure that all the
assignees are migrated