maintain user image on merging

[jpahullo]

As a user I want to see my profile image after a merging in order to be less annoying.

Acceptance conditions:

1. If both users have customized profile image, do nothing.
2. If both users have not customized profile image, do nothing.
3. If new user to maintain has no customized profile image and old user to delete has, keep that image on the new user.
4. If old user has no customized profile image, do nothing.

Notes:

• This plugin updates the profile image for the old user with a customized image to state that that user was disabled (if that is the case). So, we should do the profile image migration before updating the disabled user image.