Add Ansible role to fix Drupal file permissions

Add a new `drupal-permissions` Ansible role that resets the file and
directory permissions to match the ones described in
https://www.drupal.org/node/244924.

This will later be released to Ansible Galaxy, but for now it will
remain in this repository until the required changes are made and the
appropriate documentation is added.

Fixes #191

.gitignore

@@ -12,7 +12,7 @@
 !/phpunit.xml.dist
 !/scripts/
 !/slides/
-!/tools/
+!/tools/**
 !/web/modules/custom/**
 !/web/sites/default/environments/settings.*.php
 !/web/sites/default/settings.php