* The photo signing JWT tokens carry more fields to validate against:
* The username the token is assigned to (or '@' for anyone)
* An 'anyone' boolean for widely public images, such as for the chat room
and public profile pages.
* A short filename hash of the image in question (whether a Photo or a
CommentPhoto) - so that the user can't borrow a JWT token from the chat
room and reveal a different picture.
* Refactored where the VisibleAvatarURL function lives, to avoid a cyclic
dependency error.
* Originally: (*models.User).VisibleAvatarURL(other *models.User)
* Now: (pkg/photo).VisibleAvatarURL(user, currentUser *models.User)
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