Automatically tag animated_gif and animated_png when a post is edited.
Add them back if the user tries to remove them from an animated post,
or remove them if the user tries to add them to a non-animated post.
Before we added these tags at upload time, but it was possible for users
to remove them after upload, or to incorrectly add them to non-animated
posts. They were added at upload time because we couldn't afford to open
the file and parse the metadata on every tag edit. Now that we save the
metadata in the database, we can do this.
This also makes it so you can't tag ugoira on non-ugoira files.
Known bug: it's possible to have an animated GIF where every frame is
identical. Post #3770975 is an example. This will be detected as an
animated GIF even though visually it doesn't appear to be animated.
Fixes#4041: Animated_gif tag not added to preprocessed uploads
d5981754c4