Import bug: User images are not retained properly

[ErisDS]

When importing from LTS, some user images are not retained properly. Have seen this on a couple of occasions where an image has gone missing and originally assumed it was gravatar not being imported.

However, it is actually the gravatar step on save running and in some cases, overwriting the user's image. E.g. if I import a user with my personal email address which is registered with gravatar, but that already has a non-gravatar image set, then my non-gravatar image is replaced with gravatar.

Note: this behaviour is a bit unexpected on a normal user save, not just import, but for now I'm going to fix the importer bug only. This is because users are currently migrating from LTS.

I will then raise a separate issue to spec different behaviour for user images with gravatar.