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.

app/controllers/media_assets_controller.rb

@@ -0,0 +1,8 @@
+class MediaAssetsController < ApplicationController
+  respond_to :json, :xml
+
+  def index
+    @media_assets = authorize MediaAsset.visible(CurrentUser.user).paginated_search(params, count_pages: true)
+    respond_with(@media_assets)
+  end
+end

app/logical/upload_service.rb

@@ -104,6 +104,13 @@ class UploadService
       p.uploader_id = upload.uploader_id
       p.uploader_ip_addr = upload.uploader_ip_addr
       p.parent_id = upload.parent_id
+      p.media_asset = MediaAsset.new(
+        md5: upload.md5,
+        file_ext: upload.file_ext,
+        file_size: upload.file_size,
+        image_width: upload.image_width,
+        image_height: upload.image_height,
+      )
 
       if !upload.uploader.can_upload_free? || upload.upload_as_pending?
         p.is_pending = true

app/models/media_asset.rb

@@ -0,0 +1,7 @@
+class MediaAsset < ApplicationRecord
+  def self.search(params)
+    q = search_attributes(params, :id, :created_at, :updated_at, :md5, :file_ext, :file_size, :image_width, :image_height)
+    q = q.apply_default_order(params)
+    q
+  end
+end

app/models/post.rb

@@ -40,6 +40,7 @@ class Post < ApplicationRecord
   belongs_to :approver, class_name: "User", optional: true
   belongs_to :uploader, :class_name => "User", :counter_cache => "post_upload_count"
   belongs_to :parent, class_name: "Post", optional: true
+  has_one :media_asset, foreign_key: :md5, primary_key: :md5
   has_one :upload, :dependent => :destroy
   has_one :artist_commentary, :dependent => :destroy
   has_one :pixiv_ugoira_frame_data, :class_name => "PixivUgoiraFrameData", :dependent => :destroy

app/policies/media_asset_policy.rb

@@ -0,0 +1,5 @@
+class MediaAssetPolicy < ApplicationPolicy
+  def index?
+    true
+  end
+end