uploads: use storage manager to distribute files.

Refactors the upload process to pass around temp files, rather than
passing around file paths and directly writing output to the local
filesystem. This way we can pass the storage manager the preview /
sample / original temp files, so it can deal with storage itself.

* Change Download::File#download! to return a temp file.

* Change DanbooruImageResizer and PixivUgoiraConverter to accept/return
  temp files instead of file paths.

* Change Upload#generate_resizes to return temp files for previews and samples.

* Change Upload#generate_resizes to generate ugoira .webm samples
  synchronously instead of asynchronously.

app/logical/downloads/file.rb

@@ -3,9 +3,9 @@ module Downloads
     class Error < Exception ; end
 
     attr_reader :data, :options
-    attr_accessor :source, :original_source, :downloaded_source, :file_path
+    attr_accessor :source, :original_source, :downloaded_source
 
-    def initialize(source, file_path, options = {})
+    def initialize(source, options = {})
       # source can potentially get rewritten in the course
       # of downloading a file, so check it again
       @source = source
@@ -14,9 +14,6 @@ module Downloads
       # the URL actually downloaded after rewriting the original source.
       @downloaded_source = nil
 
-      # where to save the download
-      @file_path = file_path
-
       # we sometimes need to capture data from the source page
       @data = {}
 
@@ -35,12 +32,13 @@ module Downloads
     def download!
       url, headers, @data = before_download(@source, @data)
 
-      ::File.open(@file_path, "wb") do |out|
-        http_get_streaming(uncached_url(url, headers), out, headers)
-      end
+      output_file = Tempfile.new(binmode: true)
+      http_get_streaming(uncached_url(url, headers), output_file, headers)
 
       @downloaded_source = url
       @source = after_download(url)
+
+      output_file
     end
 
     def before_download(url, datums)
@@ -91,7 +89,8 @@ module Downloads
           end
 
           if res.success?
-            return
+            file.rewind
+            return file
           else
             raise Error.new("HTTP error code: #{res.code} #{res.message}")
           end