Implement UI for deleting hosted image attachments

[jrobbins]

When a feature entry is edited to remove the only link to an attachment, that attachment should be marked deleted in datastore.

I think this requires two steps:

1. On the server, when a POST is made to update a field of type attachments, it should compare the old value to the new value and delete any attachments that are no longer linked.
2. In the UI, the user could simply delete the line of text that has that URL. We could also offer an X icon on the thumbnails.
3. In the UI, we should probably have some kind of confirmation step because users might accidentally delete an attachment.

Once the attachment is marked deleted, requests for that attachment will give a 404. There is already logic for that in get_attachment() in attachments.py.

[Akashbayzid]

__