Remove need for image proxy

[phinze]

Back when we added image uploading, Rails didn't have a lot of options for generating image thumbnails. That's why we're using an image proxy in production (https://imagekit.io). It works well, but for the sake if having fewer third party dependencies and making it easier for multiple developers to recreate the production environment locally (and since our image uses are very simple), it would be nice to update how we handle item images:

• Keep uploads working as-is
• Configure Rails to track whether a thumbnail was generated already (may need a migration for this)
• Replace image tags with ones that point directly to the assets if they exist on S3
• If they don't, use the redirect method so that Rails will generate the thumbnail on the fly and then store it in S3
• Remove image kit code from the app

[phinze]

Relevant resource on Rails-native image handling w/ variants: https://pragmaticstudio.com/tutorials/using-active-storage-in-rails