Convert backend of SubProject model to Eloquent

[williamjallen]

This PR is part of our ongoing effort to switch all of our models to Laravel's Eloquent ORM library. The legacy SubProject model has been turned into an adapter for the Eloquent-based model, using the same approach taken in #1556.

[josephsnyder]

@williamjallen, looks like it could use a rebase when you have the time:

Merge queue failed due to phpstan:

------ ----------------------------------------------------------------------- 
9:   Line   app/cdash/app/Model/SubProject.php                                     
9:  ------ ----------------------------------------------------------------------- 
9:          Ignored error pattern #^Method                                         
9:          CDash\\Model\\SubProject\:\:AddDependency\(\) throws checked           
9:          exception Illuminate\\Database\\Eloquent\\ModelNotFoundException but   
9:          it's missing from the PHPDoc @throws tag\.$# in path                   
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:          Ignored error pattern #^Method CDash\\Model\\SubProject\:\:Delete\(\)  
9:          throws checked exception                                               
9:          Illuminate\\Database\\Eloquent\\ModelNotFoundException but it's        
9:          missing from the PHPDoc @throws tag\.$# in path                        
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:          Ignored error pattern #^Method CDash\\Model\\SubProject\:\:Delete\(\)  
9:          throws checked exception LogicException but it's missing from the      
9:          PHPDoc @throws tag\.$# in path                                         
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:          Ignored error pattern #^Method CDash\\Model\\SubProject\:\:Fill\(\)    
9:          throws checked exception                                               
9:          Illuminate\\Database\\Eloquent\\ModelNotFoundException but it's        
9:          missing from the PHPDoc @throws tag\.$# in path                        
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:          Ignored error pattern #^Method                                         
9:          CDash\\Model\\SubProject\:\:GetDependencies\(\) throws checked         
9:          exception Illuminate\\Database\\Eloquent\\ModelNotFoundException but   
9:          it's missing from the PHPDoc @throws tag\.$# in path                   
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:          Ignored error pattern #^Method                                         
9:          CDash\\Model\\SubProject\:\:RemoveDependency\(\) throws checked        
9:          exception Illuminate\\Database\\Eloquent\\ModelNotFoundException but   
9:          it's missing from the PHPDoc @throws tag\.$# in path                   
9:          /cdash/app/cdash/app/Model/SubProject.php was not matched in reported  
9:          errors.                                                                
9:  ------ ----------------------------------------------------------------------- 
9: 
9: 
9:  [ERROR] Found 6 errors                                                         
9: 
  9/230 Test   #9: php_static_analysis .............................................***Failed   83.28 sec