uploads: refactor to simplify ugoira-handling and replacements:

* Make it so replacing a post doesn't generate a dummy upload as a side effect.
* Make it so you can't replace a post with itself (the post should be regenerated instead).
* Refactor uploads and replacements to save the ugoira frame data when
  the MediaAsset is created, not when the post is created. This way it's
  possible to view the ugoira before the post is created.
* Make `download_file!` in the Pixiv source strategy return a MediaFile
  with the ugoira frame data already attached to it, instead of returning it
  in the `data` field then passing it around separately in the `context`
  field of the upload.

app/models/media_asset.rb

@@ -1,5 +1,7 @@
 class MediaAsset < ApplicationRecord
   has_one :media_metadata, dependent: :destroy
+  has_one :pixiv_ugoira_frame_data, class_name: "PixivUgoiraFrameData", dependent: :destroy, foreign_key: :md5, primary_key: :md5
+
   delegate :metadata, to: :media_metadata
   delegate :is_non_repeating_animation?, :is_greyscale?, :is_rotated?, to: :metadata
 
@@ -136,6 +138,7 @@ class MediaAsset < ApplicationRecord
       self.image_height = media_file.height
       self.duration = media_file.duration
       self.media_metadata = MediaMetadata.new(file: media_file)
+      self.pixiv_ugoira_frame_data = PixivUgoiraFrameData.new(data: media_file.frame_data, content_type: "image/jpeg") if is_ugoira?
     end
 
     def delete_files!

app/models/pixiv_ugoira_frame_data.rb

@@ -1,5 +1,6 @@
 class PixivUgoiraFrameData < ApplicationRecord
-  belongs_to :post
+  belongs_to :post, optional: true, foreign_key: :md5, primary_key: :md5
+  belongs_to :media_asset, foreign_key: :md5, primary_key: :md5
 
   serialize :data
   before_validation :normalize_data, on: :create
@@ -9,7 +10,7 @@ class PixivUgoiraFrameData < ApplicationRecord
   end
 
   def self.search(params)
-    q = search_attributes(params, :id, :data, :content_type, :post)
+    q = search_attributes(params, :id, :data, :content_type, :post, :md5)
     q.apply_default_order(params)
   end
 

app/models/post.rb

@@ -40,7 +40,7 @@ class Post < ApplicationRecord
   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
+  has_one :pixiv_ugoira_frame_data, class_name: "PixivUgoiraFrameData", foreign_key: :md5, primary_key: :md5
   has_many :flags, :class_name => "PostFlag", :dependent => :destroy
   has_many :appeals, :class_name => "PostAppeal", :dependent => :destroy
   has_many :votes, :class_name => "PostVote", :dependent => :destroy

app/models/upload.rb

@@ -19,7 +19,7 @@ class Upload < ApplicationRecord
     end
 
     def validate_integrity(record)
-      if record.media_file.is_corrupt?
+      if record.file.is_corrupt?
         record.errors.add(:file, "is corrupted")
       end
     end
@@ -55,7 +55,7 @@ class Upload < ApplicationRecord
     end
 
     def validate_video_duration(record)
-      if !record.uploader.is_admin? && record.media_file.is_video? && record.media_file.duration > MAX_VIDEO_DURATION
+      if !record.uploader.is_admin? && record.file.is_video? && record.file.duration > MAX_VIDEO_DURATION
         record.errors.add(:base, "video must not be longer than #{MAX_VIDEO_DURATION.seconds.inspect}")
       end
     end
@@ -105,10 +105,6 @@ class Upload < ApplicationRecord
   end
 
   concerning :FileMethods do
-    def media_file
-      @media_file ||= MediaFile.open(file, frame_data: context.to_h.dig("ugoira", "frame_data"))
-    end
-
     def delete_files
       # md5 is blank if the upload errored out before downloading the file.
       if is_completed? || md5.blank? || Upload.exists?(md5: md5) || Post.exists?(md5: md5)