Duplicate e-mail addresses: U4-315, U4-2147, U4-304

- Allows for duplicate e-mail addresses in member section by opt-in
(requiresUniqueEmail="false" in
web.config/membership/providers/"UmbracoMembershipProvider").
- Fixes issues with backoffice not checking for duplicate emails when
changing member after creation.
- Lower cases email addresses upon creating member and when searching to
prevent duplicates.
- Added Member.GetMembersByEmail to get alle members with specific email
Merge branch '6.0.7' of https://github.com/bokmadsen/Umbraco-CMS into bokmadsen-6.0.7