Create DBPassword and refactor Member to use it

[emteknetnz]

While working on Implement DBField validation
there' an opportunity to split of some of the password functionality from the Member DataObject so that it could be used elsewhere, and also to just make the code a little cleaner.

The would allow passwords to be programatically updated e.g. $someDataObject->Password = 'lorem-ipsum'; would handle encryption, unlike now where it's done in places like Member::onBeforeWrite()

Implementation could look something like this:

• Create a class DBPassword extends DBComposite
• It includes the $db fields 'Password', 'Salt' and 'PasswordEncryption'
• Logic is moved from Member to DBPassword e.g. encryption

We would probably also want to update PasswordValidator to extend FieldValidator and use that to validate the password before encyption, though that would be an api-break

[GuySartorelli]

We would probably also want to update PasswordValidator to extend FieldValidator and use that to validate the password before encyption, though that would be an api-break

We're already breaking the API for PasswordValidator in CMS 6 so assuming this targets the 6.0.0 release that's totally fine.