Fix this: https://danbooru.donmai.us/uploads.json?search[media_assets][md5]=b83daa7f1ae7e4127b1befd32f71ba10 failing with an ActiveRecord::StatementInvalid error. The bug was that for a `has_many through: ...` association, like `has_many :media_assets, through: :upload_media_assets`, we weren't joining on the associated table properly so we ended up generating invalid SQL.
Add a join table that allows multiple media assets (images or videos) to be attached to uploads. This is for a future ability to upload multiple files at once.