settings: Add setting to control how animated images are played.

Previously animated images were automatically played in the
message feed of the web app.

Now that we have still thumbnails available for them, we can add a new
personal setting, "web_animate_image_previews", which controls how the
animated images would be played in the web app message feed -- always
played, on hover, or only in the image viewer.

Fixes #31016.

zerver/views/realm.py

@@ -638,6 +638,7 @@ def update_realm_user_settings_defaults(
         Annotated[int, check_int_in_validator(UserProfile.USER_LIST_STYLE_CHOICES)]
     ]
     | None = None,
+    web_animate_image_previews: Literal["always", "on_hover", "never"] | None = None,
     email_address_visibility: Json[
         Annotated[int, check_int_in_validator(UserProfile.EMAIL_ADDRESS_VISIBILITY_TYPES)]
     ]