Provide a template for Reference entity to facilitate theming

Problem/Motivation

Hello,

I tried to render a book reference entity type in another view mode. But this do not render anything.

Steps to reproduce
- Create a new view mode for reference entity type. Let's call it "Teaser"
- Enable this view mode and configure it to show only some few fields
- Create a view and choose to render entities based on the Teaser mode

When rendering reference entity in another view mode, fields are rendered separatly, field by field, without a global wrapper with attributes. It's not possible to theme each reference.

Proposed resolution

After looking at the code, it looks like bibcite_entity do not provide a default template for bibcite_reference and there is not some basic preprocessing to provide variables to this template. There is only a custom template bibcite_reference_table (note the simple _ where as should be bibcite_reference__table - with 2 __ if this template was referering to the view mode table for bibcite_reference entity).

Looks like the issue is from the ReferenceViewBuilder

protected function getBuildDefaults(EntityInterface $entity, $view_mode) {
    $build = parent::getBuildDefaults($entity, $view_mode);

    switch ($view_mode) {
      case 'table':
        $build['#theme'] = 'bibcite_reference_table';
        break;
    }

    return $build;
  }

and of course because bibcite_entity module do not implement a theme for bibcite_reference entity and do not provide the corresponding template.

Remaining tasks

- Agreed on the fix
- Write a pacth

User interface changes

None

API changes

None

Data model changes

None