Fixing up scenarios for managing page permissions Behat tests.

Relies on changes from silverstripe-behat-extension:

silverstripe-behat-extension/pull/15
silverstripe-behat-extension/pull/14
silverstripe-behat-extension/pull/13
silverstripe-behat-extension/pull/12
NEW STEP: Pages editable by and renamed manage page permissions feature file
Merge pull request #904 from srizzling/is/cms/892

Added new step Pages can be editable by
Reload "Settings" form on save (fixes #895)

Regression from 920da0d1202ec403c193a0b282f5379bc415c287
ValidationException support in CMS "Settings" UI
More specific Behat assertion for page editability