Add support for many_many_extraField in YAML

Format is

RelationName:
  - =>Obj.name:
    ExtraFieldName: "Foo"
Reset FailedLoginCount on successful password reset
Update preview.md

The JavaScript entwine example contains an extra jQuery wrapper
Merge pull request #2915 from torleif/patch-2

Update preview.md
Merge pull request #2911 from silverstripe-iterators/pulls/reset-failedlogincount

Reset FailedLoginCount on successful password reset
Merge pull request #2894 from wilr/3.1-manymanyyaml

Add support for many_many_extraField in YAML