post replacements: refactor and fix tests.

* Move replacement tests from test/unit/upload_service_test.rb to
  test/functional/post_replacement_controller_test.rb
* Move UploadService::Replacer to PostReplacementProcessor.
* Fix a minor bug where if you used the API to replace a post with a file,
  the replacement would fail unless you passed an empty string for the
  replacement_url.

app/logical/post_replacement_processor.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+class PostReplacementProcessor
+  attr_reader :post, :replacement
+
+  def initialize(post:, replacement:)
+    @post = post
+    @replacement = replacement
+  end
+
+  def process!
+    media_file = get_file_for_upload(replacement.replacement_url, nil, replacement.replacement_file&.tempfile)
+
+    if Post.where.not(id: post.id).exists?(md5: media_file.md5)
+      replacement.errors.add(:base, "Duplicate: post with md5 #{media_file.md5} already exists")
+      return
+    end
+
+    if media_file.md5 == post.md5
+      media_asset = post.media_asset
+    else
+      media_asset = MediaAsset.upload!(media_file)
+    end
+
+    if replacement.replacement_file.present?
+      canonical_url = "file://#{replacement.replacement_file.original_filename}"
+    else
+      canonical_url = Sources::Strategies.find(replacement.replacement_url).canonical_url
+    end
+
+    replacement.replacement_url = canonical_url
+    replacement.file_ext = media_asset.file_ext
+    replacement.file_size = media_asset.file_size
+    replacement.image_height = media_asset.image_height
+    replacement.image_width = media_asset.image_width
+    replacement.md5 = media_asset.md5
+
+    post.md5 = media_asset.md5
+    post.file_ext = media_asset.file_ext
+    post.image_width = media_asset.image_width
+    post.image_height = media_asset.image_height
+    post.file_size = media_asset.file_size
+    post.source = replacement.final_source.presence || replacement.replacement_url
+    post.tag_string = replacement.tags
+
+    rescale_notes(post)
+
+    replacement.save!
+    post.save!
+
+    post.update_iqdb
+  end
+
+  def rescale_notes(post)
+    x_scale = post.image_width.to_f  / post.image_width_was.to_f
+    y_scale = post.image_height.to_f / post.image_height_was.to_f
+
+    post.notes.each do |note|
+      note.rescale!(x_scale, y_scale)
+    end
+  end
+
+  def get_file_for_upload(source_url, referer_url, file)
+    return MediaFile.open(file) if file.present?
+    raise "No file or source URL provided" if source_url.blank?
+
+    strategy = Sources::Strategies.find(source_url, referer_url)
+    raise NotImplementedError, "No login credentials configured for #{strategy.site_name}." unless strategy.class.enabled?
+
+    strategy.download_file!
+  end
+end