CRM-21088: Changing drupal calls to backdrop calls

[laryn]

Overview

The Backdrop class contains a number of drupal_* function calls.

If Backdrop is running without compatibility mode enabled, drupal_* calls will not be valid. They should be changed to backdrop_* calls.

I've included/updated some methods that are in DrupalBaseClass as well to override and do the same for them.

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• CRM-21088: Backdrop class contains drupal_ calls that should be changed

[laryn]

@totten See if this one (and this one) help with the Angular pages. I'm now able to view the System status report on a Backdrop site.