Fix temp files generated during the upload process not being cleaned up quickly enough. This included
downloaded files, generated preview images, and Ugoira video conversions.
Before we relied on `Tempfile` cleaning up files automatically. But this only happened when the
Tempfile object was garbage collected, which could take a long time. In the meantime we could have
hundreds of megabytes of temp files hanging around.
The fix is to explicitly close temp files when we're done with them. But the standard `Tempfile`
class doesn't immediately delete the file when it's closed. So we also have to introduce a
Danbooru::Tempfile wrapper that deletes the tempfile as soon as it's closed.
* Add "general" rating.
* Rename "safe" rating to "sensitive".
* Change safe mode to include both rating:s and rating:g.
* Treat rating:safe as a synonym for rating:sensitive.
* Link "howto:rate" in the post edit form.
For small thumbnails, use 180x180 thumbnails scaled down to 150x150.
This is so we can get rid of 150x150 images and just use 180x180 for
both small and medium size thumbnails.
Also fix RSS feeds, XML sitemaps, and Discord embeds to use 360x360
thumbnails instead of 150x150 thumbnails.
* Add a `DiscordSlashCommand.register_slash_commands!` method to register
all slash commands with the Discord API.
* Allow registering global commands.
* Refactor slash commands to use class attributes for the command
name, description, and options.
