[UX][D8] Convert user pictures to an actual Image Field

[jenlampton]

We need to get rid of user pictures and use build in fields instead. See related Drupal issues:

• Convert user pictures to Image Field
• Fix user_picture field creation

Also for reference, here is the change record.

Follow-ups:

• We'll need to lock the picture field
• This will introduce a performace regression, so we should keep an eye on this issue.

[mikemccaffrey]

We need to lock the user picture field so it doesn't get deleted.

Actually, it seems like when the user picture is a regular field, we won't need to rely on any special code to display it in the templates. And we should probably let people delete the field if they wish.

What we really need to do is make sure that any user-picture-specific functions that are called don't error once the field is converted and possibly deleted by the user.

[klonos]

...And we should probably let people delete the field if they wish.

I think so too.

[klonos]

...this is actually part of #378

[klonos]

This will consolidate things and reduce custom code, but it is also a UX issue. When a user profile picture has been added, there is a "Delete picture" checkbox, with a "Check this box to delete your current picture." help text, which is really odd and counter-intuitive (there is also no information re the filename etc.):

Whereas regular image fields provide a "Remove" button instead: