Regenerate posts asynchronously using a delayed job.
Regenerating a post can be slow because it involves downloading the
original file, regenerating the thumbnails, and redistributing the new
thumbnails back to the image servers. It's better to run this in the
background, especially if a user is trying to regenerate posts in bulk.
The downside is there's no notification to the user when the regeneration
is complete. You have to check the modactions log to see when it's finished.
* Remove the PostRegeneration model. Instead just use a mod action
to log when a post is regenerated.
* Change it so that IQDB is also updated when the image samples are
regenerated. This is necessary because when the images samples are
regenerated, the thumbnail may change, which means IQDB needs to be
updated too. This can happen when regenerating old images with
transparent backgrounds where the transparency was flattened to black
instead of white in the thumbnail.
* Only display one "Regenerate image" option in the post sidebar, to
regenerate both the images and IQDB. Regenerating IQDB only can be
done through the API. Having two options in the sidebar is too much
clutter, and it's too confusing for Mods who don't know the difference
between an IQDB-only regeneration and a full image regeneration.
* Add a confirm prompt to the "Regenerate image" link.
