added support for nested module on the dashboard

[zeezo887]

Description

This allows nested modules to appear in the activities, drafts, count, and search sections of the dashboard.

Usage:

Add the module to the dashboard.modules array of twill/config.php. e.g.

\App\Models\IssueArticle::class => [
    'name' => 'issues.articles',
    'label' => 'Articles',
    'label_singular' => 'Article',
    'count' => true,
    'activity' => true,
    'draft' => true,
    'search' => true,
    'search_fields' => ['title', 'description'],
    'parentRelationship' => 'issue'
]

Related Issues

Fixes #1697