Add MediaAsset model.

A MediaAsset represents an image or video file uploaded to Danbooru. It
stores the metadata associated with the image or video. This is to work
on decoupling files from posts so that images can be uploaded separately
from posts.

db/migrate/20210901230931_create_media_assets.rb

@@ -0,0 +1,19 @@
+class CreateMediaAssets < ActiveRecord::Migration[6.1]
+  def change
+    create_table :media_assets do |t|
+      t.timestamps null: false, index: true
+
+      t.string :md5, null: false, index: true, unique: true
+      t.string :file_ext, null: false, index: true
+      t.integer :file_size, null: false, index: true
+      t.integer :image_width, null: false, index: true
+      t.integer :image_height, null: false, index: true
+    end
+
+    reversible do |dir|
+      dir.up do
+        execute "INSERT INTO media_assets (created_at, updated_at, md5, file_ext, file_size, image_width, image_height) SELECT created_at, created_at AS updated_at, md5, file_ext, file_size, image_width, image_height FROM posts ORDER BY id ASC"
+      end
+    end
+  end
+end