[bugfix] Don't allow entry of alt text for avatar/header if avatar/header not set

[tsmethurst]

Right now it's a big confusing because the text field to enter an image description for avatar/header is still enabled in the settings panel even when there's no avatar or header to set a description on. We should make this less confusing by disabling the header description field if there's no header image set, and disabling the avatar description field if there's no avatar image set.

Related: #3452