Build out a URI Adapter to allow uploading an avatar via a url

Currently only really accessible via the API. The UriAdapter creates a
tempfile from a url and gives a ActionDispatch::HTTP::UploadedFile back
to the controller to process as normal.
This will help a lot in being able to transfer avatar urls from another
app without monkey patching a lot of discourse code.

app/services/uri_adapter.rb

@@ -0,0 +1,64 @@
+# For converting urls to files
+class UriAdapter
+
+  attr_reader :target, :content, :tempfile, :original_filename
+
+  def initialize(target)
+    raise Discourse::InvalidParameters unless target =~ /^https?:\/\//
+
+    @target = URI(target)
+    @original_filename = ::File.basename(@target.path)
+    @content = download_content
+    @tempfile = TempfileFactory.new.generate(@original_filename)
+  end
+
+  def download_content
+    open(target)
+  end
+
+  def copy_to_tempfile(src)
+    while data = src.read(16*1024)
+      tempfile.write(data)
+    end
+    src.close
+    tempfile.rewind
+    tempfile
+  end
+
+  def file_size
+    content.size
+  end
+
+  def build_uploaded_file
+    return if (SiteSetting.max_image_size_kb * 1024) < file_size
+
+    copy_to_tempfile(content)
+    content_type = content.content_type if content.respond_to?(:content_type)
+    content_type ||= "text/html"
+
+    ActionDispatch::Http::UploadedFile.new( tempfile: tempfile,
+                                            filename: original_filename,
+                                            type: content_type
+                                            )
+  end
+end
+
+# From https://github.com/thoughtbot/paperclip/blob/master/lib/paperclip/tempfile_factory.rb
+class TempfileFactory
+  ILLEGAL_FILENAME_CHARACTERS = /^~/
+
+  def generate(name)
+    @name = name
+    file = Tempfile.new([basename, extension])
+    file.binmode
+    file
+  end
+
+  def extension
+    File.extname(@name)
+  end
+
+  def basename
+    File.basename(@name, extension).gsub(ILLEGAL_FILENAME_CHARACTERS, '_')
+  end
+end