AJAX friendly responses for Security class

Final work around issue #1802 - creating templates for complex layout
operations and removing HTML from this controller.
Use correct config variable name in encrypt_password

Use correct config variable name in encrypt_password
Fixes https://github.com/silverstripe/sapphire/issues/1709
Merge pull request #1 from silverstripe/master

Syncing back because I suck at Git
Updating docs for Security template changes

See https://github.com/silverstripe/sapphire/pull/1807
Merge pull request #1807 from willmorgan/master

Allow easier control of Security templates with AJAX, etc.