Time to delete recently read is not configurable.

Patch works great, but I have some comments about it.

1. Configuration link is added with *links.action.yml, which is not the best way to do it. It is better to create configuration link as a tab with *.links.task.yml.
2. In RecentlyReadConfigForm change:

  protected function getEditableConfigNames() {
    return [
      'recentlyread.config',
    ];
  }

with

  protected function getEditableConfigNames() {
    return [
      'delete_config',
      'delete_time',
    ];
  }

There is no configuration object with 'recentlyread.config' name.
3. Fix coding standards in buildForm method of RecentlyReadConfigForm class (replace array() with []).

one more suggestion:

add cron task to cleanup expire logs.

@zterry95 expire logs are already deleted on cron with this patch. The patch looks good.

Great work @nkoporec and thanks @icurk for testing.