Add functionality to delete avatar

[snytkine]

The link to "delete image" is already on user profile page
Now need to implement functionality
If use has uploaded avatar - delete it
if user has external avatar - delete it from USER record
if user does not have own or external avatar then add b_nogrvtr flag and use that flag to NOT show gravatar? The problem with that is that user cannot undo this.
If user still wants to somehow show their avatar they need to just upload a new image.

[epinapala]

I will start working on implementing this feature :). Thanks!