CRM-19303 - Drupal - Restore getCiviSourceStorage()

CRM-19303 - Drupal - Restore getCiviSourceStorage()

This partially reverts changes made under CRM-19303:
 * The changes to `getDefaultFileStorage()` are preserved
 * The changes to `getCiviSourceStorage()` are reverted
 * To avoid conflicts, `parseDrupalSiteName()` has been split into the older `parseDrupalSiteNameFromRoot()` and the newer `parseDrupalSiteNameFromRequest()`.
   (IMHO, `parseDrupalSiteNameFromRequest()` is more correct, but `parseDrupalSiteNameFromRoot()` is more compatible with existing deployments).

As discussed in [PR #10513](https://github.com/civicrm/civicrm-core/pull/10513#issuecomment-310506829),
we should stop trying so hard to autodetect these things.  We'll treat the
auto-detection stuff as legacy, and should shift toward a simpler/flatter arrangement
which encourages paths/URLs to be stored in `civicrm.settings.php`.