Merge branch '28080-system-checks' into 'master'

changelogs/unreleased/28080-system-checks.yml

lib/system_check.rb

lib/system_check/app/active_users_check.rb

lib/system_check/app/database_config_exists_check.rb

lib/system_check/app/git_config_check.rb

lib/system_check/app/git_version_check.rb

lib/system_check/app/gitlab_config_exists_check.rb

lib/system_check/app/gitlab_config_up_to_date_check.rb

lib/system_check/app/init_script_exists_check.rb

lib/system_check/app/init_script_up_to_date_check.rb

lib/system_check/app/log_writable_check.rb

lib/system_check/app/migrations_are_up_check.rb

lib/system_check/app/orphaned_group_members_check.rb

lib/system_check/app/projects_have_namespace_check.rb

lib/system_check/app/redis_version_check.rb

lib/system_check/app/ruby_version_check.rb

lib/system_check/app/tmp_writable_check.rb

lib/system_check/app/uploads_directory_exists_check.rb

lib/system_check/app/uploads_path_permission_check.rb

lib/system_check/app/uploads_path_tmp_permission_check.rb